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FXUS02 KWBC 212027PMDEPDExtended Forecast DiscussionNWS Weather Prediction Center College Park MD426 PM EDT Wed Apr 21 2021Valid 12Z Sat Apr 24 2021 - 12Z Wed Apr 28 2021...Heavy Rainfall across parts of the Southeast Saturday......Overview...Progressive upper level flow is expected to evolve over the courseof the period with the main systems of interest being shortwaveenergy deepening over the Northeast/New England late this weekendand early next week while a deeper longwave trough pushes onshorethe West Coast. As it reaches the Central Plains by day 7, a moresignificant storm system could bring a variety of weather hazardsto the central U.S. by the middle of next week. Temperatures willbe variable during the period, modulated by frontal passages....Guidance/Predictability Assessment and Preferences...The 21.00Z and 21.06Z guidance showed fairly good clustering atthe starting point of the medium range. A multi-model blend of thedeterministic guidance was sufficient for the low pressure systemtracking from the mid-Mississippi Valley on day 3 /Saturday/ toNew England on day 5 /Monday/. The 06Z GFS was less favoredthough, as it was showing a faster bias compared to the otherdeterministic global models and the 00Z GFS was more agreeable andtherefore was included more in the blend. Elsewhere, the ECMWFcontinued to be a slower solution with the next trough comingthrough the West U.S. and the CMC (and to some degree the UKMET)offered a more consensus approach. Either way, once the troughejects into the Plains and cyclogenesis takes place, all of themodels actually come together for a reasonable solution for day6/7 in what may evolve into a more amplified flow pattern. TheECENS/GEFS means were used more for day 6/7 and this resulted in ablend that trended well with continuity....Weather/Hazards Highlights...Low pressure in the Tennessee Valley early Saturday will promoteshower and storms, possibly severe (see SPC), with locally heavyrain especially over parts of Georgia and through the Carolinas.The system should then lift up the East Coast as trailing northernstream energy from the Great Lakes region merges into the coastalsystem and possibly closes an upper low. Enhanced rainfall ispossible from parts of the Mid-Atlantic into New England withwindy conditions region-wide as the low deepens.In the West, an amplifying Pacific upper trough/low and surfacefront will work into the West during the weekend into early nextweek. This system will spread lower elevation rain/mountain snowfocused from the Pacific Northwest and northern Rockies southwardinto California and the Great Basin/central Rockies. The highesttotals with this system will likely be from the Pacific Northwestinto the Sierra Nevada. The majority of precipitation should belight to moderate though parts of the central West Coast andSierra Nevada could see a period of locally enhanced activity.Once the system moves to the Plains by Tuesday, potential willincrease for convectively-driven rainfall over parts of the Plainsand Mississippi Valley. See SPC for details on the potentialsevere weather threat.In the wake of the eastern system, cooler than normal temperatureswill be prevalent over the Ohio Valley and southern Mid-Atlanticinto the Northeast. An area of above normal temperatures overparts of the West/Great Basin ahead of the Pacific uppertrough/low will shift eastward across the Rockies and into thePlains by early next week and into the Northeastern 1/4 of theCONUS by midweek. Best potential for one or more days withtemperatures 10F or more above normal will extend from near theGreat Basin through the central-southern Rockies into thePlains/Midwest where temperatures may climb well into the 80s/70s,respectively. Trailing the front that crosses the West,temperatures over the region will decline to 5-15F colder thannormal by next Monday-Tuesday and the back to near or slightlyabove normal Wednesday.Taylor/FracassoHazards:- Heavy precipitation across portions of northern and centralCalFXUS21 KWNC 211758PMDTHRUS Hazards OutlookNWS Climate Prediction Center College Park MD300 PM EDT April 21 2021SYNOPSIS: Forecast mid-level low pressure is anticipated to elevate thepotential for heavy precipitation over the southeastern and south-central U.S.during the early and middle portion of the outlook period, respectively. Muchof the western U.S. looks to be hazards free with slightly elevated odds forabove-normal temperatures for much of southwestern U.S and over Alaska.HAZARDSSlight risk of heavy precipitation for portions of the Southeast, Ohio andTennessee Valleys, the Appalachians, and the Mid-Atlantic, Thu, Apr 29.Slight risk of heavy precipitation for portions of the Central and SouthernPlains, and the Middle and Lower Mississippi Valley, Sat-Tue, May 1-4.DETAILED SUMMARYFOR SATURDAY APRIL 24 - WEDNESDAY APRIL 28:https://www.wpc.ncep.noaa.gov/threats/threats.phpFOR THURSDAY APRIL 29 - WEDNESDAY MAY 05: As advertised by the dynamical modelsover the past few days, anomalous mid-level ridging continues to be featuredacross the southwestern U.S. with a downstream shortwave trough forecast tomove through the central CONUS by the outset of the week-2 period. Compared tothe GEFS, the ECMWF ensemble continues to favor a more amplified pattern withlarger coverage of positive (negative) 500-hPa height departures throughout thewestern (central) CONUS. Trends in the ECMWF ensemble regarding this shortwavetrough are of interest for hazard potential, as the today's 0z run depicts552-dam heights reaching as far south as the Midwest on day 8 (Apr 29), whichwould bring another shot of unseasonably cold temperatures and maintain heavyprecipitation potential to portions of the central and eastern U.S early in theoutlook period. By the start of May, both the GEFS and ECMWF ensemble guidanceare beginning to favor the development of anomalous troughing over the westernU.S. The 0z and 6z GEFS are particularly aggressive with this development,favoring a trough axis extending as far south as northern Mexico, which wouldhelp induce the return of surface low development over the Great Plains andincrease heavy precipitation potential by the middle portion of the outlookperiod.During late week-1 and into the start of the outlook period, surface lowpressure is anticipated to track across the upper Midwest and Great Lakes.Surface high pressure remains forecast throughout much of the western Atlantic,which favors enhanced lower-level southerly flow to advect Gulf moisture, andelevate chances for heavy precipitation over portions of the southeasternCONUS. Based on the ensemble mean precipitation totals from the GEFS and ECMWF,the highest precipitation amounts are expected over the Mississippi Valleyprior to the start of week-2, however, elevated probabilities for 24-hourprecipitation totals exceeding an inch are still depicted over parts of theSoutheast, the Ohio and Tennessee Valleys and the Appalachians on day 8.Although there is modest support in the reforecast guidance, a slight risk ofheavy precipitation remains issued for the region for Apr 29. As the frontalsystem is forecast to track across the Eastern Seaboard, below normaltemperatures are likely to prevail over the Mississippi Valley, with thepotential for frost over portions of the Midwest early in the period. Farthersouth, increased wind speeds are also possible, mainly across the Southeast.Reforecast tools indicate elevated probabilities for sustained winds exceedingthe 85th percentile in the region, however wind speeds are not expected toexceed hazard thresholds, and no wind-related hazards are issued in today'soutlook.Later in the outlook period, the 0z and 6z GEFS feature a broad area of surfacelow pressure deepening over the southern High Plains tied to the aforementionedtroughing aloft by day 10 (May 1). Deterministic solutions feature an enhancedlow-level jet setting up over the Plains to help advect Gulf moisture, andensemble members in the 0z GEFS show a good convergence of solutions at thisle