612
FXUS02 KWBC 101857
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
257 PM EDT Fri May 10 2024

Valid 12Z Mon May 13 2024 - 12Z Fri May 17 2024

...Excessive Rainfall/Flooding Threat for the Gulf Coast and
Southeast...


...Overview...

A system progressing across the southern half of the central-
eastern U.S. during the first half of the week will bring the
threat of heavy rainfall to areas near the Gulf Coast. This system
and a northern stream front will combine to spread varying
intensity of rain across areas farther north as well. Another
system currently expected to reach the Plains/east-central U.S.
after midweek may produce another broad area of rainfall with
highest totals again possible over parts of the southern tier.
Meanwhile guidance diverges significantly for flow from the eastern
Pacific into western North America by mid-late week, lowering
confidence in forecast details over that region and to some extent
downstream at that time frame.

...Guidance/Predictability Assessment...

The latest operational model guidance provided a reasonable
starting point for the first half of the forecast period including
the initial system through the Mid-South and whatever influence
from a northern stream shortwave through the Northeast early-mid
next week. After this, the guidance shows some variability in
timing and details of the next shortwave into the Central U.S.
later next week, but the greatest uncertainty at this time
continues to be out West. There are significant differences in how
much ridging will build over the West/Northwest, seeming to stem
from very uncertain shortwave energy across western Canada. The
ECMWF and CMC seem to suggest some weak troughing will suppress
ridging south and west (moreso from the ECMWF), but the last few
runs of the GFS shows a very strong and potentially blocky ridge
building over the region by next Thursday-Friday. The ensemble
means generally align with their deterministic counterparts, but
the ECMWF-initialized ML models do suggest more troughing than the
current suite of guidance. Given this, the WPC forecast trended
more towards the ECMWF with the ECENS and NAEFS (the GEFS seemed
too strong with ridging). This maintained fairly good continuity
with the previous WPC shift as well.


...Weather/Hazards Highlights...

The system progressing eastward from the southern Plains during the
first half of the week will be one generator of potentially heavy
rainfall. The best signal currently exists along and near the Gulf
Coast Monday-Tuesday, in association with a warm front lifting
north through the region and followed by the system`s trailing cold
front. The new Day 4 Excessive Rainfall Outlook maintains
continuity with a Slight Risk area extending from southeastern
Texas through southern Alabama and a portion of the Florida
Panhandle while the Day 5 ERO continues a Slight Risk area over
southern Georgia and northern Florida. Over these two days, this
region shows the best combination of instability and anomalous
moisture along with synoptic features to enhance focus. A Marginal
Risk area extends farther north into parts of the Middle
Mississippi Valley/Ohio Valley into southern Mid-Atlantic over the
two-day period, corresponding to a favorable combination of
instability, guidance signals for scattered heavy QPF maxima, and
currently wet ground conditions. There are two other areas worth
monitoring for Day 5, one over the central Plains/Midwest where a
front may stall, and another over the Northeast where a northern
stream front may enhanced rainfall. At this time there is too much
spread among the guidance to merit any risk area but that could
change if solutions gravitated toward the heavier side of the
envelope.

From early Wednesday onward, first expect some lingering rainfall
near the East Coast as the leading system departs. Then areas from
the southern Plains eastward may see yet another episode of heavy
rain as a front pushes into and through the Plains/Mississippi
Valley and the first system`s trailing front lifts back north as a
warm front. Rain of varying intensity will also be possible farther
north across parts of the central/eastern U.S.

Uncertainty with the flow aloft over the northern half of the West
by mid-late week leads to a potential range between dry conditions
with above normal temperatures or some precipitation spreading
through the Pacific Northwest/northern Rockies with temperatures
closer to normal.

Farther south, California and Nevada should be
more consistently above normal next week with a scattering of plus
10-15F anomalies. The northern Rockies/High Plains should also be
on the warm side early in the week. The clouds and heavy rainfall
potential over the Gulf Coast/Southeast early in the week should
initially keep highs below normal over that region, but
particularly Florida may warm up to a few degrees above normal by
mid to late week as another warm front lifts through. Combined with
high dew points, this may bring heat indices as high as 105-110
across southeast/southern Florida, signaling the first hazardous
heat days of the summer. Otherwise, areas to the east of the
Rockies will tend to be within a few degrees of normal for highs
while the rest of the eastern U.S. should see somewhat above
normal lows Tuesday-Friday.

Santorelli/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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