Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 PM EDT Thu May 16 2024

Valid 12Z Sun May 19 2024 - 12Z Thu May 23 2024


...Overview...

Reasonably agreeable model guidance indicates that an upper trough
and surface low should move out of the eastern U.S. but linger in
the western Atlantic through the first half of next week.
Meanwhile, mean troughing will develop in the western U.S., which
along with frontal boundaries should help develop some locally
heavy convection across north-central parts of the Plains and
Mississippi Valley toward the Great Lakes region early to midweek.
Leading shortwave energy looks to push across the northern two-
thirds of the U.S. along with a cold front Tuesday-Thursday, while
an upper low drops south into the Northwest. Cool conditions with
increasing precipitation chances are forecast for the Northwest,
including higher elevation snow.


...Guidance/Predictability Assessment...

Recent model guidance is generally agreeable with the overall
pattern, with some typical detail uncertainties. There is still
some spread with the specifics of the eastern U.S. to western
Atlantic low, but the 00Z UKMET seemed to be the main outlier with
it in showing a farther east/progressive solution more reminiscent
of models from a couple days ago, so a GFS/ECMWF/CMC blend worked
well. Farther west, models indicate a few features of note that
combine into the western U.S. trough. A small southern stream
eastern Pacific closed low Sunday will track eastward and get
absorbed likely Monday-Tuesday, while northern stream energy pivots
through the northwestern and north-central U.S. within the trough.
00Z/06Z model guidance seemed agreeable with that but the new 12Z
CMC now lingers that energy in the West longer while other models
progress it into the Midwest. The new 12Z CMC also seems like an
outlier upstream as it does not show the upper low dropping south
through the western side of the trough by midweek.

The WPC forecast used a blend of the 00/06Z GFS and 00Z ECMWF and
CMC runs early in the period, with gradually increasing
incorporation of ensemble means to about half by the end of the
period. This maintained good continuity with the previous forecast.


...Weather/Hazards Highlights...

During the Sunday-Monday night time frame covered by the Days 4-5
Excessive Rainfall Outlooks, the primary region of interest will
extend from the central Plains into the Midwest. A Plains surface
low should lift an initially stalling front northward as a warm
front while a wavy boundary should hang up for a time over the
Plains/Upper Midwest early next week while awaiting the approach of
western U.S. dynamics. There is still some scatter for details of
the placement for possibly multiple rounds of thunderstorms in this
area, but the ingredients of ample moisture/instability and lift
warrant continuing Marginal Risks in the EROs for both Days 4 and
5, with threats on Day 5 depending somewhat on how Day 4
transpires. From early Tuesday onward, expect northeastward
progression of developing low pressure to push the trailing cold
front and accompanying rain and thunderstorms of varying intensity
farther east and south. Rainfall amounts could be heavy across the
Midwest into the Great Lakes region, but other than that it will
take additional time to resolve most favored areas of heaviest
rainfall in the eastern and southern U.S. for the Tuesday-Thursday
time frame.

Aside from the north-central U.S., there is some concern Sunday
for locally heavy rain in the Southeast to southern Mid-Atlantic
with the departing low pressure system, but without enough of a
signal to warrant a risk area. Then in the West, initial upper
troughing will support some precipitation over northern areas with
scattered activity possibly reaching as far south as the Great
Basin. Some snow will be possible in the high elevations of the
Rockies, along with high winds possible on Sunday. Approach/arrival
of another upper low as currently advertised by midweek or so
would produce somewhat more organized precipitation at that time.
Changes in snow levels will depend on the depth of the upper low.

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. Some daily records for highs/warm lows will
be possible. Southern Florida may also experience hazardous heat
into Sunday with highs locally up to 5-10F above normal and max
heat index values reaching 105-110F, followed by continued above
normal but slightly less extreme readings.

The forecast pattern will favor below average highs over the
Northwest for most of next week, and including the northern Plains
by early next week. The warm sector from the High Plains into the
Great Lakes/New England early in the week should see highs up to
10-15F or so above normal. Cold frontal progression should bring
temperatures closer to normal over most of the eastern half of the
country by mid-late week. Clouds and possibly lingering pockets of
rainfall should keep highs on the cool side over the Mid-Atlantic
during Sunday-Monday.


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