134
FXUS02 KWNH 150700
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 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







$$