377
FXUS02 KWBC 140659
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Tue May 14 2024

Valid 12Z Fri May 17 2024 - 12Z Tue May 21 2024

...Another southern tier heavy rainfall threat during the latter
half of the week...


...Overview...

Downstream from a reorienting eastern Pacific upper ridge, guidance
is generally suggesting flat to broadly cyclonic mean flow late
this week into the weekend, followed by a tendency toward more
upper troughing over the West early next week. There are still a
lot of lower-predictability detail questions about flow rounding
the ridge and reaching the lower 48. There is also considerable
spread for the details/timing of an upper shortwave and associated
surface system crossing the eastern half of the country late this
week into the weekend. Continue to expect the heaviest convective
emphasis to be across the southern tier late this week into the
weekend, with some areas of less extreme but potentially meaningful
rainfall farther north.

...Guidance/Predictability Assessment...

Over recent days the dynamical models/ensembles and machine
learning (ML) models have been showing difficulty in resolving
important but medium to smaller scale details within progressive
Northeast Pacific into lower 48/southern Canada flow, and then how
potentially better defined western troughing may evolve
(influencing when an upper low drifting toward California
accelerates inland). There has been plenty of spread and run-to-run
variability over the East as well, with significant differences for
timing/details of shortwave energy and associated surface
reflection late this week onward plus what the larger scale flow
will look like by early next week.

Based on comparisons of 12Z/18Z guidance, a composite of
operational model solutions provided the most palatable starting
point. At the start of the period Friday the 12Z ML models offered
more support toward flatter flow over the Northwest than the
GFS/GEFS. By mid-late period, the ML models generally leaned toward
the cluster of 12Z ECMWF/CMC and their ensemble means which were
more progressive than the GFS/GEFS for the system crossing the East
and moving into the Atlantic, and showed less pronounced digging
south of an upper low that may reach southwestern Canada and the
northwestern U.S. (but with spread for incorporation of the
southern stream Pacific into California upper low) This majority
cluster also suggested that by next Tuesday there should be some
degree of upper troughing over eastern North America with ridging
over the Plains, in contrast to a flat ridge reaching the East with
southwesterly flow over the Plains. Thus the forecast leaned more
toward the 12Z ECMWF-ECens/CMC-CMCens scenario. The one
reservation was that incorporation of the Pacific/California low
could have been a little slow.

New 00Z guidance continues to highlight the volatility of the
pattern, with the UKMET/CMC trending toward the GFS over the
Northwest as of early Friday. Then the average of guidance
trends slower with more separation for the eastern U.S. upper
shortwave late week into the weekend, while the ECMWF has flipped
somewhat in the GFS direction for the pattern over the East by next
Tuesday.

...Weather/Hazards Highlights...

Another southern tier convective event should be in progress at the
start of the period early Friday. The Day 4 Excessive Rainfall
Outlook that covers the Friday-Friday night time frame continues to
depict a Slight Risk area that extends from Louisiana into Alabama,
with some adjustment from continuity to reflect latest guidance.
This area will be sensitive to additional rainfall given wet soil
conditions. Frontal/wave focus with available moisture and
instability along with potential for repeat rains may enhance
totals. The surrounding Marginal Risk area accounts for some
potential of a convective axis hanging back even farther to the
west (still most pronounced in GFS runs). Farther north, latest
ECMWF/GFS/CMC/ICON runs are starting to cluster toward a potential
axis of less extreme but locally heavy rainfall over the Ohio
Valley in association with a front affecting the region. Short-term
rainfall should lead to somewhat wetter soil than initially
observed. The Day 4 ERO introduces a Marginal Risk area to account
for the developing guidance cluster. By Day 5 Saturday, the only
coherent area of potentially heavy rainfall is over portions of the
Southeast as the southern tier surface front/waviness continue
onward. A Marginal Risk provides a reasonable starting point given
the guidance scatter within this region. Confidence is much lower
for rainfall coverage and totals farther northward over the East.

Otherwise, 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. Some areas of rainfall may develop
over portions of the northern-central Plains into the Midwest by
the weekend or early next week depending on shortwave
energy/surface details, with low confidence for specifics of
intensity at any particular location. Portions of the West may
begin to see scattered precipitation by the weekend and early next
week with the tendency toward more upper troughing and arrival of
the southern stream upper low.

The southern two-thirds of the West will likely see above normal
temperatures continue through the weekend with some areas seeing
highs 10-15F or so above normal. Northern areas will already be
moderating close to normal Friday-Sunday while additional lowering
of heights aloft over the West early next week should bring
temperatures toward normal over the remainder of the West. Some of
the western warmth will extend into portions of the Plains and
Mississippi Valley with plus 5-15F anomalies. The Florida
Peninsula and South Texas should see several days with highs up to
5-10F above normal, in a warm pattern to the south of a couple
fronts expected to focus upon reaching more northern parts of these
states. Some daily records for highs/warm lows will be possible.
These temperatures may bring heat indices as high as 105-110
degrees for south Florida and upwards to 115 degrees for south
Texas, highlighting a threat of hazardous heat.

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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