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FXUS02 KWBC 151917

PMDEPD



Extended Forecast Discussion

NWS Weather Prediction Center College Park MD

317 PM 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 consistent 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...



Even as the period begins Saturday, there are some model/ensemble 

differences across the east-central U.S. with the upper trough and 

weak surface low. Among an overall slower trend in the guidance 

over the past day or two, GFS runs remain on the slower side of the

envelope in tracking these features east compared to non-NCEP 

models and AI/ML models. The CMC may be a good middle ground in 

position as the ECMWF is on the eastern side. Into next week, there

is an increasing model signal for this energy and the surface low 

to linger in the western Atlantic near the Outer Banks or so. The 

most recent 12Z ECMWF now shows this as well after more progressive

earlier runs.



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. Multiple rounds of shortwave 

energy should arrive into the Northwest during the weekend into 

next week, 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. At least generally, models show a couple of rounds of 

central U.S. surface low development Monday and Tuesday with a 

northeast track by Wednesday but with low- moderate confidence in 

the details. Timing and strength of upstream energy diving through 

the west side of the trough next week is also uncertain. The 12Z 

ECMWF now clusters with the GFS runs in showing a closed upper low 

nearing or atop the Pacific Northwest by midweek.



The WPC forecast was based on a blend of the 06Z GFS and 00Z ECMWF

and CMC early in the period, and gradually increased the ensemble 

means component to half by Day 6 and over half Day 7 as the spread 

increased. This generally maintained good continuity with the 

previous forecast though with a slightly slower/lingering western 

Atlantic low.





...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, even as 

simple as ensemble probabilities of exceeding an inch, 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 energies ejecting from the mean trough 

and developing surface low pressure systems. 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. As some guidance indicates 

maximum amounts from Nebraska/southeast South Dakota to Minnesota 

while some are well south from Kansas to Missouri, continue to 

depict no risk area in the ERO at this point but expect that a 

Marginal Risk would be needed if a more coherent guidance signal 

arises. Another round of perhaps heavier rainfall amounts is likely

over portions of the northern half of the Plains and Mississippi 

Valley into Monday-Tuesday of next week, after the ERO period. 

Meanwhile, 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. Highs near 

or over 100 degrees could stretch farther north across the southern

High Plains at times. 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 extreme 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 during the period. Clouds and rainfall should keep 

highs on the cool side over the Mid-Atlantic during the weekend.





Tate/Rausch





Additional 3-7 Day Hazard information can be found on the WPC medium 

range hazards outlook chart at:

https://www.wpc.ncep.noaa.gov/threats/threats.php



WPC medium range 500mb heights, surface systems, weather grids, 

quantitative precipitation forecast (QPF), excessive rainfall 

outlook (ERO), winter weather outlook (WWO) probabilities, heat 

indices, and Key Messages can be accessed from:



https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif

https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif

https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html

https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml

https://www.wpc.ncep.noaa.gov/#page=ero

https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4

https://www.wpc.ncep.noaa.gov/heat_index.shtml

https://www.wpc.ncep.noaa.gov/#page=ovw





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