Experiences with 0–36-h Explicit Convective Forecasts with the WRF-ARW Model

31 Pages·2008·7.94 MB·English

JUNE2008 WEISMAN ET AL. 407
Experiences with 0–36-h Explicit Convective Forecasts with the WRF-ARW Model
MORRIS L. WEISMAN, CHRISTOPHER DAVIS, WEI WANG, KEVIN W. MANNING, AND JOSEPH B. KLEMP
NationalCenterforAtmosphericResearch,*Boulder,Colorado
(Manuscriptreceived11January2007,infinalform24October2007)
ABSTRACT
Herein,asummaryoftheauthors’experienceswith36-hreal-timeexplicit(4km)convectiveforecasts
withtheAdvancedResearchWeatherResearchandForecastingModel(WRF-ARW)duringthe2003–05
spring and summer seasons is presented. These forecasts are compared to guidance obtained from the
12-kmoperationalEtaModel,whichemployedconvectiveparameterization(e.g.,Betts–Miller–Janjic´).The
results suggest significant value added for the high-resolution forecasts in representing the convective
systemmode(e.g.,forsqualllines,bowechoes,mesoscaleconvectivevortices)aswellasinrepresentingthe
diurnalconvectivecycle.However,noimprovementcouldbedocumentedintheoverallguidanceastothe
timingandlocationofsignificantconvectiveoutbreaks.Perhapsthemostnotableresultistheoverallstrong
correspondencebetweentheEtaandWRF-ARWguidance,forbothgoodandbadforecasts,suggestingthe
overriding influence of larger scales of forcing on convective development in the 24–36-h time frame.
SensitivitiestoPBL,landsurface,microphysics,andresolutionfailedtoaccountforthemoresignificant
forecasterrors(e.g.,completelymissingorerroneousconvectivesystems),suggestingthatfurtherresearch
isneededtodocumentthesourceofsucherrorsatthesetimescales.Asystematicbiasisalsonotedwith
theYonseiUniversity(YSU)PBLscheme,emphasizingthecontinuingneedtorefineandimprovephysics
packagesforapplicationtotheseforecastproblems.
1. Introduction vective-system-resolvingforecastsduringthewarmsea-
son,measuredobjectivelyorsubjectively,ascompared
Convective weather remains a significant challenge to coarser-resolution simulations using convective pa-
fornumericalweatherpredictionsystems,andisrecog- rameterizations [e.g., operational 12-km North Ameri-
nizedasamajorcontributortopoorwarmseasonquan- can Mesoscale Model (NAM, formerly the Eta
titative precipitation forecasting (QPF). During the Model)].Amoregenericgoalwassimplytoestablisha
2003 Bow Echo and Mesoscale Convective Vortex better sense of the predictability of significant convec-
(MCV)Experiment(BAMEX;Davisetal.2004;Done tiveoutbreaksoverthenextdiurnalcycle(18–36-hpe-
etal.2004),andagainfor2004and2005,36-hreal-time riod),andtobetterunderstandthefactorsthatmightbe
forecastswereconducteddailywiththeAdvancedRe- limiting this predictability within current numerical
search Weather Research and Forecasting Model forecastmodels.Certainly,thepotentialapplicationsof
(WRF-ARW, or simply ARW; Skamarock et al. 2005) suchconvective-resolvingforecastscangowellbeyond
using a 4-km horizontal grid resolution over increas- regional QPF, including explicit prediction of severe
ingly large domains centered on the central United surface winds, hail, localized flash flooding, and even
States (Fig. 1). The overall goal of these exercises was perhapstornadoes.Inthisregard,itisalsoimportantto
todetermineifthereisanyincreasedskillinsuchcon- considerwhetherthereissufficientvalueaddedtosuch
higher-resolution guidance to warrant running such
computationallyexpensiveforecastsoperationally.This
*TheNationalCenterforAtmosphericResearchissponsored latterquestioncannotbeansweredbythisinitialstudy,
bytheNationalScienceFoundation. but hopefully this study will help build a foundation
that will contribute to such discourse.
Attemptsatusingcoarser-resolutionmodelsoforder
Corresponding author address: Morris L. Weisman, NCAR,
10kmwithparameterizedconvectiontoguideconvec-
P.O.Box3000,Boulder,CO80307-3000.
E-mail:[email protected] tive weather forecasting have generally not fared very
DOI:10.1175/2007WAF2007005.1
©2008AmericanMeteorologicalSociety
WAF2007005
408 WEATHER AND FORECASTING VOLUME23
FIG.1.Domainsusedforthe2003–054-kmreal-timeWRF-ARWforecasts.
well,providingoneoftheprimarymotivationsforthis radarassimilationwithinnumericalmodels.Inparticu-
work.WilsonandRoberts(2006)foundthatthe10-km lar, he noted “Weather prediction is the principal rea-
RapidUpdateCycle(RUC10)3-hforecastsofprecipi- son for the support we are given by our fellow citi-
tationinitiationwerecorrectonly44%ofthetimedur- zens....Ibelieveitistimeforconvective-stormscien-
ing the International H O Project (IHOP), allowing a tists to apply our knowledge to this purpose and to
2
tolerance of 250 km in space and about 3 h intime. subjectourproductstoitsdiscipline....Thetaskisnot
TheyalsofoundthattheRUC10wasgenerallynotsuc- trivial....” Indeed, this review helped motivate and
cessful in forecasting the evolution and motion of the guide the development of the Center for the Analysis
larger, more intense storm systems, presumably be- and Prediction of Storms (CAPS; e.g., Droegemeier
cause of the lack of gust fronts. Gallus et al. (2005) 1990,1997),whoseformalmissionwasto“demonstrate
similarly document the inability of 10-km versions of thepracticabilityofstormscalenumericalweatherpre-
the Eta Model, ARW, and the fifth-generation Penn- diction and to develop, test, and validate a regional
sylvania State University–National Center for Atmo- forecast system appropriate for operational, commer-
spheric Research Mesoscale Model (MM5) with a cial, and research applications.”
variety of physics and initializations to forecast a well- Progress along these lines has indeed been notable
documentedderechoevent12–24hinadvance.Coarse- over the past decade. For example, using a 6-km grid
resolution models with parameterized convection also with explicit convection over Lake Michigan, Fowle
have known deficiencies in their representation of the and Roebber (2003) demonstrated enhanced skill at
diurnal cycle of convection as well as the related pre- forecastingconvectiveoccurrence,coverage,andover-
cipitation episodes over the United States (e.g., Davis allconvectivemode(e.g.,linear,multicellular,andiso-
et al. 2003; Carbone et al. 2002). lated) out to 2 days as compared to coarser-resolution
Lilly(1990)offersaninsightfulreviewoftheoppor- forecasts with parameterized convection. Xue et al.
tunitiesandchallengesassociatedwithexplicitconvec- (2001, 2003) describe a successful 8–18-h simulation of
tive forecasting, with respect to the inherent predict- a line of tornadic storms in Arkansas on 21 January
ability of convection as well as the proper representa- 1999 using 32-, 6-, and 2-km nested grids, starting at
tion of convective processes and the potential uses of 1200UTC.Theyfoundgenerallygoodagreementasto
Fig 1live4/C
JUNE2008 WEISMAN ET AL. 409
TABLE1.Modelspecifications.
Initialand Radiation
WRF Domain Vertical boundary (longwave/ Land Micro-
Year version (km) levels conditions Turbulence shortwave) PBL surface physics
2003 1.3 2000(cid:1)2000 35 40-kmEta3-h 2DSmagorinski RapidRadiative YSU OSU Lin
forecast TransferModel
(RRTM)/
Dudhia
2004 1.3 2800(cid:1)2600 35 40-kmEta3-h 2DSmagorinski RRTM/Dudhia YSU OSU Lin
forecast
2005 2.0.3.1 3900(cid:1)3000 35 40-kmEta3-h 2DSmagorinski RRTM/Dudhia YSU Noahand WSM6
forecast HRLDAS
the number of storms, the rotational characteristics of eralhoursinadvancebyinitiatinganested2-kmgrid18
thestorms,thespeedanddirectionofstormmovement, hintoacoarser-resolutionmodelforecastinitializedat
the upscale organization from isolated cells into lines, 0000 UTC the night before.
and, subsequently, a bow-shaped pattern. They did These previous studies clearly show the potential to
noteupto2-hdelaysinstorminitiationinsomeregions explicitlyforecasteventhedetailsofconvectionseveral
andpositionerrorsofupto40km.The6-kmgridcor- hours in advance. In the present study, we build upon
rectly predicted the general characteristics of the these experiences by extending the time frame of the
storms in the late afternoon and evening, but with a forecastperiodtoconsideroneandahalfdiurnalcycles
timing delay of as much as 4 h, and with less accuracy ofconvectiveactivity(18–36h),andalsobyemploying
forcelldetailsandrotationalcharacteristics.Theyalso much larger forecast domains, thus emphasizing more
observed that “the 32-km grid played a key role in re- of the regional convective forecasting issues. In addi-
producing accurately the large-scale environment that tion, we consider a much larger sample size (232 cases
fed the convective storms in this region but by itself it over3yr),whichrepresentsabroaderspectrumofcon-
was incapable of providing specific, detailed guidance vective scenarios. In contrast, no radar assimilation
on the timing and location of precipitation or the type techniques are applied.
and characteristics of the systems that produced the Wepresenthereonlyarepresentativesampleofour
precipitation.” forecastexperiencestodate,asmoredetailedanalyses
Xue et al. (2003) further describe a successful 2–3-h are still on going. Even at this early stage, however,
prediction of tornadic cells observed over Fort Worth, valuable lessons have been learned that will hopefully
Texas, on 28–29 March 2000, using the Advanced Re- guideothersimilarefforts.Webegininsection2witha
gional Prediction System (ARPS) with a 3-km grid briefdiscussionofthemodelspecificsforthisexercise,
resolutionembeddedwithinalarger9-kmdomain,and followedinsection3bycomparisonsbetweenforecast-
by employing various radar assimilation strategies. derivedradarreflectivitiesandobservedradarcompos-
Kong et al. (2006) considered the use of ensembles to ites to establish the capabilities of the model at repro-
predictthissametornadicthunderstormcomplex,using ducing the range of observed convective phenomena
nestedgridsof24,6,and3km.Theensemblesshowed criticalforsuchforecastapplications.Wealsocompare
both qualitative and quantitative improvements rela- this guidance to that offered by the 12-km Eta during
tivetothedeterministiccontrolforecasts.However,the thistimeperiod.Insection4wepresentsomeprelimi-
evolutions of convection at 24 and 6 km were vastly narysubjectiveverificationsoftheoverallcapabilityof
different from and inferior to those at 3 km. For the theexplicitconvectiveforecastsrelativetothecoarser-
3-kmgridforecasts,Kongetal.(2007)foundthatboth resolutionEtaguidance,followedinsection5byadis-
theassimilationofDopplerradardataandshorter(1–2 cussion of forecast sensitivities to various physics op-
h) forecast lead times improved the ensemble precipi- tions, such as PBL and microphysical representation,
tation forecasts. Similarly Xue and Martin (2006a, b) as well as resolution. In section 6, we discuss the im-
successfully simulated convective initiation along the proved ability of such resolutions to reproduce ob-
dryline2–3hinadvanceon24May2002duringIHOP, servedprecipitationepisodesanddiurnaltendencies.In
using ARPS at 1-km grid resolution nested within a section 7, we discuss the forecaster feedback we re-
3-km domain. Finally, Roebber et al. (2002) were able ceived during these exercises. Finally, in section 8, we
to reproduce many of the supercellular characteristics offer some summary comments and directions for fu-
ofthe3May1999tornadooutbreakinOklahomasev- ture work.
410 WEATHER AND FORECASTING VOLUME23
FIG.2.(a)ObservedNOWRADcompositereflectivityat0400UTC4Jun2005comparedto(b)4-hforecastmaximumcolumn
reflectivityfromWRF-ARWat0400UTC,startingfromaninitialstatewithzeroprecipitation.
2. Simulation design ever,suggestthatexplicitconvectivetriggeringatsuch
gridresolutionsissufficientinmostcases,ifnotactually
TheARWmodelspecificationsforeachyearofthese a bit overdone at times.
exercises are summarized in Table 1. The choice of 4 The model was initialized at 0000 UTC using the
kmforthehorizontalgridresolutionwasbasedbothon 40-km NAM/Eta (hereafter Eta) analysis, with the
the practical constraints of completing the simulations boundaryconditionsupdatedona3-hourlyintervalus-
overnight as well as recent studies that suggest that 4 ing the Eta Model forecasts. The ARW output was
km is nominally sufficient to represent mesoscale con- generally available by 1400 UTC (0800 local time), so
vective systems explicitly without the need for param- that the forecast guidance would be available to Na-
eterization (e.g., Weisman et al. 1997). Such grid reso- tional Weather Service (NWS) forecasters in time to
lutions, however, are still insufficient for representing address the afternoon/evening convective forecast
individual convective cells and their attendant severe problem.Thechoicetomovethewesternboundaryof
weather(e.g.,Bryanetal.2003).Thus,inevaluatingthe the forecast domain from the Continental Divide in
presentmodelcapabilities,weareinterestedintheabil- 2003 to Nevada for the 2004 and 2005 seasons (Fig. 1)
ity to forecast the location and timing of regions of was based on a noticeably weaker than observed diur-
convection and the structure and evolution of meso- nal maximum in convective activity over the highest
scaleconvectivesystemsasopposedtothecharacteris- terrain in 2003 (e.g., see section 6). We speculate that
tics of individual convective elements. Use of a 4-km thisdeficiencywasrelatedtotheinabilityofthemodel
grid over a single large domain, rather than using se- toproperlyreproducethelarge-scaleorographiccircu-
lective embedded high-resolution windows, avoids the lationassociatedwiththeRockyMountains,butamore
problemofhavingtochooseappropriatewindowsona detaileddiscussionofthisresultisbeyondtheintended
day-to-daybasisandalsoavoidsthepotentialproblems scope of the present study.
associated with mismatched model physics across Anotherdesignfactorthatwewereconcernedabout
nested-domain boundaries [e.g., explicit convection on was the use of a cold-start initialization at 0000 UTC,
the inner grid versus parameterized convection on the whenactiveconvectionwasoftenalreadyoccurring.As
outer grid; e.g., Warner et al. (1997); Warner and Hsu showninFig.2,however,observedconvectivesystems
(2000)]. The lack of a convective trigger function for a andotherprecipitationfeatureswereoftenreasonably
4-kmgridresolutionwasinitiallyapointofconcernin reproducedbythemodelwithin3–5hofinitialization.
designing these experiments. Our experiences, how- Indeed, we may have been aided in this regard by our
Fig 2live4/C
JUNE2008 WEISMAN ET AL. 411
choiceofinitializingat0000UTC,whensurface-based founditmostconvenienttocomparethemaximumcol-
convectivepotentialwasatitsgreatest,makingiteasier umnreflectivityasderivedfrommodelvariablestothe
togenerateconvectioninregionsofresolvedlow-level observed maximum reflectivity obtained from WSI
convergence. This result is also consistent with the ki- Corporation National Operational Weather Radar
netic energy spectral analyses of Skamarock (2004), (NOWRAD)mosaics.Radarreflectivityforthemodel
whoshowedmoregenerallythatsmall-scalestructures output was derived based on the mixing ratios of hy-
in such simulations are effectively spun up over the drometeor species (e.g., rain, snow, graupel), as de-
initial 6–12 h. This provides some confidence that our scribed by Koch et al. (2005).
results for the (cid:2)12 h forecast periods were not unduly Another goal is to consider whether there is any
influenced by our simple cold-start procedure. value added in the present high-resolution simulations
ThebasicphysicspackagesincludedtheYonseiUni- as compared to coarser-resolution simulations using
versity (YSU) boundary layer scheme (Hong et al. convective parameterization. For this purpose, we
2006),theOregonStateUniversity(OSU)landsurface chose to compare the 4-km ARW results primarily to
model (Chen and Dudhia 2001), and the Lin micro- the 12-km operational Eta, which was used to specify
physics scheme [derived from the original scheme de- the initial state as well as the boundary conditions for
scribed in Lin et al. (1983)]. For 2005, the Lin scheme the present ARW runs, and which was also readily
was replaced by the WRF single-moment six-class mi- available in real time and quite familiar to forecasters.
crophysicsscheme(WSM6;HongandLim2006)based In addition, 12-km versions of ARW were run for the
on sensitivity testing that suggested the WSM6 repro- select cases described in section 3 [using the Betts–
duced stratiform precipitation somewhat better while Miller–Janjic´ (BMJ) convective parameterization
also slightly reducing a known high precipitation bias. scheme, as is also used by the Eta], though, to help
Also for 2005, we employed the National Center for clarifywhichdifferencesmaybespecifictothediffering
Atmospheric Research (NCAR) High-Resolution modelformulationsversusresolution.Forinstance,we
LandDataAssimilationSystem(HRLDAS;Chenetal. have noted that ARW generally produces finer-scale
2007) to help improve the representation of soil mois- detailsintheprecipitationfieldscomparedtotheEtaat
ture in the initial state. Although these variations rep- a given grid resolution, primarily due to the differing
resentpotentiallysignificantchangesinthemodelcon- numerical techniques employed (e.g., Skamarock
figuration over the 3 yr considered herein, retesting of 2004). However, we have found that such differences
a representative set of cases has confirmed that they aresmallcomparedtothegenericdifferencesbetween
resultedinonlyrelativelysmallchangesinoverallfore- the12-kmparameterizedand4-kmexplicitapproaches,
cast accuracy, and thus we feel confident in discussing reflecting the enhanced ability of 4-km resolutions to
the results of all 3 yr together. begin to represent the convective phenomena of inter-
Further sensitivity testing on specific cases also con- est (e.g., Done et al. 2004). More specific comparisons
sidered the use of the Miller–Yamada–Janjic´ (MYJ) between ARW and Eta, run at 10-km horizontal reso-
PBL scheme (Janjic´ 2001), the Thompson microphysi- lution for a variety of model physics and initial condi-
cal scheme (Thompson et al. 2006), and initialization tionconfigurations,canbefoundinGallusetal.(2005).
using the analysis from the 25-km resolution RUC- Incomparingthe4-kmARW(explicit)versus12-km
nativecoordinatedata(e.g.,sigma–thetainthevertical) Eta and ARW (parameterized) convective forecasts,
asobtainedfromtheNationalCentersforEnvironmen- wemustacceptthat,inmanyrespects,wearecompar-
tal Prediction (NCEP) operational site. Some of the ingapplesandoranges.Forthe4-kmARWmodel,we
moresignificantsensitivitieswillbediscussedfurtherin emphasize the comparison between the observed and
section 5. model-derived reflectivities. For the 12-km Eta and
ARW,theonlyrelatedoutputreadilyavailableinreal
time was 6-h total precipitation (reflectivity could be
3. Overall results
calculated for the explicit portion of the precipitation,
The primary goal of this study is to document the but not for the parameterized portion, which is where
abilityof4-kmgridresolutionsimulationstorepresent our primary interest lies). However, for the present
and forecast the characteristics of convective weather purposes, such a comparison between reflectivity and
systems.Assuch,itismostappropriatetocomparethe total rainfall helps to highlight the different character-
model output directly to Next-Generation Doppler isticsoftheguidanceavailablefromthedifferingmodel
(NEXRAD) radar observations, which represents the configurations.Equivalent6-hprecipitationtotalsfrom
method most readily available to forecasters for docu- the4-kmARW,however,willalsobeincludedforease
menting convective weather. For this purpose, we ofcomparisonwiththeEtaresultsandtheobserved6-h
412 WEATHER AND FORECASTING VOLUME23
FIG.3.ObservedNOWRADcompositereflectivityat(a)0000and(b)0600UTC10Jun2003comparedto(c)24-and(d)30-h
forecastmaximumcolumnreflectivityfromWRF-ARW,forforecastsinitializedat0000UTC9Jun2003.
NCEP stage IV (ST4) precipitation analyses. The mode of convective organization, such as squall lines,
3-hourly precipitation totals were acquired for the Eta bow echoes, and mesoscale convective vortices (e.g.,
for some of the post-real-time analyses and are dis- Done et al. 2004), information that has generally not
cussed in section 6 below in addressing convective cli- beenpreviouslyavailabletoforecasters.Anexampleof
matologies. oneofthebetterforecastsispresentedinFig.3,which
All in all, we were surprised at the ability of these depictsanintensebow-echosystemoverIowaandNe-
explicit forecasts not only to predict the timing and braskapredicted24–30hinadvanceon10June2003.In
location of many significant convective outbreaks out addition to reproducing a bow-shaped system propa-
to36h,butalsotoproviderealisticguidanceastothe gatingsoutheastward,the4-kmforecastswerealsoable
Fig 3live4/C
JUNE2008 WEISMAN ET AL. 413
FIG. 4. (a) Maximum column reflectivity, (b) surface flow field, (c) surface equivalent potential temperature, and (d) northwest–
southeastverticalcrosssection,aslocatedin(a),ofreflectivityandground-relativeflowfromthe30-hWRF-ARWforecast,validat
0600UTC10Jun2003.
to generate a realistic surface outflow (Fig. 4b) and 100km.Also,ratherthanonelargebowsegmentevolv-
associated cold pool (Fig. 4c), along with a rear-inflow ing as in the simulation, the actual event was charac-
jet (Fig. 4d), as was indeed observed with this system terized by two smaller bow systems linked together.
(e.g., Davis et al. 2004). Such convectively generated The24–30-hforecastsfromthe12-kmEtaandARW
mesoscale features are largely absent in coarser- models for this case were quite similar to each other
resolution simulations that employ convective param- (Figs. 5a and 5b), highlighting eastern Nebraska and
eterization.Still,theupscalegrowthofconvectivecells western Iowa and Minnesota for heavy precipitation
into an organized convective system is several hours (withabitmorestructureevidentintheARWforecast,
earlier than observed, and is off in location by about as discussed previously), but no guidance is offered as
Fig 4live4/C
414 WEATHER AND FORECASTING VOLUME23
FIG.5.The0000–0600UTCaccumulatedprecipitation(mm)for10Jun2003fromthe(a)24–30-hEtaforecast,(b)24–30-h12-km
ARWforecast,(c)NCEPST4analysis,and(d)24–30-h4-kmARWforecast.
to the probable mode of convective organization (un- Anexamplefrom12June2003(Fig.6)demonstrates
lesssuchinformationcouldbeinferredindirectlyfrom theabilityofsuchforecaststocaptureavarietyofcon-
thelarger-scaleparameterssuchasCAPEandvertical vectivescenariosinthesameforecastperiod,fromthe
wind shear). The equivalent 6-h precipitation totals regenerationofconvectioninassociationwithanMCV
from the 4-km ARW forecast (Fig. 5d) depict a simi- over southern Illinois and Kentucky, to a bow-shaped
larly broad area of heavy precipitation (although sig- system propagating southward through Arkansas, to
nificantly overforecast), but additionally depict indi- convection developing in the plains from the Dakotas
vidualprecipitationstreaksembeddedwithinthelarger extendingdowntoColorado.Onemajorfailureforthis
pattern, which represents the propagation of the case, however, was the lack of any convection forecast
smaller-scaleconvectiveelementsthatarebeginningto in central Oklahoma and Texas, where, in reality, a
be resolved at the 4-km grid resolution. The observed large,organizedsqualllineoccurred.Subsequentanaly-
precipitation for this case (Fig. 5c), however, depicts sisofthiscasesuggeststhatthetriggeringforthissquall
several distinct swaths of precipitation from Kansas line originated from a subtropical upper-tropospheric
through the Dakotas, with two primary swaths in Ne- wavethatwaspoorlyrepresentedintheARWsimula-
braska, associated with the two bowing segments de- tion.Theguidancefromthe12-kmEtaandARWmod-
picted in Fig. 3b. els for this case (Figs. 7a and 7b) were also somewhat
Fig 5live4/C
JUNE2008 WEISMAN ET AL. 415
FIG.6.ObservedNOWRADcompositereflectivityat(a)0000and(b)0600UTC12Jun2003comparedto(c)24-and(d)30-h
forecastmaximumcolumnreflectivityfromWRF-ARW,forforecastsinitializedat0000UTC11Jun2003.
mixed, generally capturing the region of widespread Oneoftheconcernsinusinga4-kmgridresolutionis
precipitation associated with the MCV as well as the whether such resolutions would be sufficient to distin-
convectionovertheplains,butgenerallymisrepresent- guish between outbreaks of more isolated severe con-
ing the characteristics of the two distinct convective vective storms versus organized convective systems. A
systems in Oklahoma and Arkansas. Indeed, the con- case from 30 May 2004 (Fig. 8) highlights such con-
vective system in Arkansas turned out to be the pri- cerns. By 0000 UTC, a fairly widespread outbreak of
mary precipitation producer on this day (Fig. 7c), al- severe convection has developed in Nebraska extend-
thoughthe4-kmARWagainsignificantlyoverforecast ing northward into the Dakotas, with more isolated
the amount of precipitation for this and other corre- (tornadic)supercells(highlightedbyarrows)extending
sponding events on this day (Fig. 7d). southward along the dryline in central Oklahoma
Fig 6live4/C
416 WEATHER AND FORECASTING VOLUME23
FIG.7.The0000–0600UTCaccumulatedprecipitation(mm)for12Jun2003fromthe(a)24–30-hEtaforecast,(b)24–30-h12-km
ARWforecast,(c)NCEPST4analysis,and(d)24–30-h4-kmARWforecast.
(Fig.8a).By0600UTC,thenorthernconvectionorga- icant precipitation in southeastern Kansas extending
nizes into one dominant squall line extending from into southwestern Missouri, again largely missing the
southeastern Nebraska into central Kansas, with one tornadic cells farther west in Kansas and Oklahoma.
isolated supercell remaining in northeast Oklahoma However, these more isolated cells did not produce
(Fig.8b).The4-kmARWforecastforthisevent(Figs. widespread precipitation (Fig. 9c) and, thus, might not
8c and 8d) largely captures the convective outbreak beconsideredtobeasignificanterrorwhenusingveri-
from Nebraska northward, but misses the isolated tor- fication schemes based merely on precipitation accu-
nadiccellsalongthedrylineinKansasandOklahoma. mulations.
Instead, strong convection is forecast in southeastern Finally, we present an example from 5 June 2005
KansasandspreadingintosouthernMissouri.Interest- (Fig.10)inwhichanintensesqualllinedevelopsalong
ingly,the6-haccumulatedprecipitationfromthe12-km the dryline of central Oklahoma and Kansas by 0000
Eta and ARW model forecasts valid at 0600 UTC UTC,andproceedseastwardoverthenext6h,result-
(Figs. 9a and 9b) similarly depict widespread heavy ing in two primary bow-shaped systems over western
precipitation from Nebraska extending northeast- IllinoisandsoutheasternOklahomaby0600UTC.The
wardintoMinnesota,withanadditionalareaofsignif- 4-km ARW captures the early convection quite well
Fig 7live4/C