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FXUS02 KWBC 150658

PMDEPD



Extended Forecast Discussion

NWS Weather Prediction Center College Park MD

258 AM EDT Wed May 15 2024



Valid 12Z Sat May 18 2024 - 12Z Wed May 22 2024



...Southern tier heavy rainfall threat possibly lingering into the

weekend over the Southeast...





...Overview...



Behind an upper trough and surface system progressing eastward from

the Mississippi Valley Saturday onward, guidance shows a system

crossing southern Canada and the northern tier of the lower 48

during the weekend. Then guidance is conistent in principle

regarding establishment of what may become a fairly amplified mean

trough aloft over the West. While a lot of embedded details are

still uncertain, the forecast pattern evolution should support a

transition of rainfall emphasis from the South/East to start the

weekend toward development from the northern half of the Plains 

into the Midwest and continuing eastward between Sunday and the

middle of next week. Meanwhile the western trough will likely 

bring cooler temperatures to the region and at least some scattered

precipitation, including high elevation snow in the northern 

Rockies.





...Guidance/Predictability Assessment...



Confidence in particular details continues to be below average

based on guidance variability and spread over recent days. This 

includes eventual clustering toward different particular model 

solutions depending on the feature, and some discrepancies between 

machine learning (ML) guidance and dynamical solutions.



For the system progressing eastward from the Mississippi Valley

Saturday onward, the past 24-36 hours of guidance has generally

gravitated toward the slower side of the envelope as represented by

prior GFS runs. This contrasts with the ML models which previously

liked the progressive side and as of the 12Z cycle were still

somewhat faster than the dynamical model average. Adding to the

uncertainty, in varying ways latest GFS-GEFS/CMC runs have the 

upper trough getting stuck near the East Coast into next week. The 

new 00Z ECMWF remains more progressive than the GFS-GEFS/CMC.



With the Saturday-Sunday southern Canada/northern tier U.S. system

resolved fairly well, the next feature of interest is the 

developing western U.S. upper trough. The first bundle of shortwave

energy should arrive into the Northwest during the weekend with one

or more upstream features possibly helping to deepen/reinforce the

overall trough next week. This brings into play the typical low-

predictability issue of when/how individual shortwaves will eject

from the mean trough and generate well-defined surface waves east

of the Rockies. Adding to this, a southern stream upper low will

likely reach into the Southwest by early in the week and may

contribute to low pressure development downstream. 



Within the western trough, the 12Z ML models leaned to the deep

side of the dynamical spread and with a greater signal for an

embedded upper low (which the GFS/ECMWF depict at varying times).

Downstream there is currently decent clustering from the dynamical

guidance toward low pressure organizing over the Plains by next 

Tuesday (with some waviness already taking shape Monday) and then 

northeastward progression into Wednesday with the GFS/GEFS/CMC 

over the Great Lakes and the ECMWF/ECens/CMCens faster to varying 

degrees. 12Z ML models generally favored the faster scenario, and 

the new 00Z ECMWF has adjusted faster as well.



Based on the array of 12Z/18Z guidance, the updated forecast

started with an operational model composite early in the period

followed by a trend to half means (18Z GEFS/12Z ECens) with

lingering input from the 12Z ECMWF/12Z CMC/18Z GFS in order of more

to less weight.





...Weather/Hazards Highlights...



As of Day 4 Saturday-Saturday night, the upper trough/surface

system moving through the East will spread an area of rainfall

across the southern two-thirds or more of the East. The Excessive

Rainfall Outlook covering this period depicts a Marginal Risk area

extending from northern Florida into the central Appalachians and

parts of the Mid-Atlantic. There should be favorable moisture and

instability within this area but so far the guidance has not

clustered particularly well for heaviest activity. The primary

region of interest for the possibility of an eventual Slight Risk

upgrade would be in the vicinity of northern Florida into southern

Alabama/Georgia if guidance shows better clustering, given wet

antecedent conditions and what signals currently exist for high

rain rates.



Some rainfall could linger along parts of the East Coast into 

Sunday if the slower side of the envelope verifies for the system 

affecting the region, but details remain very uncertain. Meanwhile

the greater rainfall emphasis will shift into the central

Plains/Midwest region around the start of the week, in association

with shortwave energy ejecting from the mean trough and developing

surface low pressure. This activity should continue to the east

with time, but with uncertainty for specifics. During the Day 5

(Sunday-Sunday night) ERO period, guidance shows mixed signals

regarding the magnitude/location of convective development within

the central Plains/Midwest region. Preference at this time is to

depict no risk area but with the understanding that a more coherent

guidance signal in future runs could merit a Marginal Risk. Portions

of the West, especially northern areas, may begin to see scattered

precipitation by the weekend or early next week and continuing into

midweek as upper troughing settles over the region. Some

precipitation may be in the form of snow in the high elevations of

the northern Rockies.



Expect South Texas to see multiple days of hazardous heat during

the period with highs persistently running 10-15F above normal with

max heat index values possibly reaching at least 110F. Southern

Florida may also experience hazardous heat during the weekend with

highs locally up to 5-10F above normal and similar max heat index

values, followed by continued above normal but slightly less extrem

readings. Both areas may see daily record highs.



The southern two-thirds of the West will be quite warm on Saturday

but then the developing upper trough will lead to a cooling trend,

with northern parts of the West seeing near to below normal highs

from the weekend onward and areas to the south trending to a mix of

slightly above/below normal highs. Areas from the central/southern

Rockies into the Great Lakes will tend to see above normal

temperatures (generally most anomalous over the High Plains) during

the period. Clouds and rainfall should keep highs on the cool side

over the Mid-Atlantic during the weekend.



Rausch











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