211
FXUS02 KWBC 160659
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Thu May 16 2024

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


...Overview...

At least for the moment, most guidance seems to be starting to
gain somewhat better agreement and consistency for the larger scale
evolution during the period. An initial western trough plus an
incoming shearing-out southern stream Pacific low should eject
eastward Tuesday onward while a Bering Sea into Gulf of Alaska
upper low may drop into the Northwest U.S. by next Thursday to
reinforce the mean trough over the West. The leading ejecting
western energy should support Plains into eastern Canada surface
low pressure with a trailing front expected to push through most of
the eastern half of the lower 48 Tuesday-Thursday. This will
support areas of locally heavy convection across parts of the
Plains/Midwest/Great Lakes with activity continuing east/southeast
with time. The Northwest should trend colder with gradually
increasing precipitation, including high elevation snow in the
northern Rockies. A system near the East Coast on Sunday may linger
for a while just offshore the Southeast/southern Mid-Atlantic
coast.


...Guidance/Predictability Assessment...

While there are ongoing detail uncertainties and/or a stray
solution or two for certain aspects of the forecast, continuity of
general ideas from the 12Z/18Z guidance majority plus 00Z model
trends of prior less agreeable solutions have incrementally
improved confidence for the general ideas of most forecast
features.

That said, the forecast of the system along/offshore the southern
half of the East Coast continues to be a challenge for the
guidance. Once again the average of dynamical guidance is a little
slower than most 12Z ECMWF-initialized machine learning (ML)
models, while the dynamical solutions are still shuffling around a
little for timing. The 12Z/18Z dynamical cluster was a little
slower than 12 hours prior, but the 00Z UKMET has strayed to the
fast side.

Behind this system, there is a more coherent theme today that the
combination of ejecting initial western U.S. troughing, an incoming
southern stream Pacific low that shears out underneath or just
behind the ejecting trough, and a lingering southern Canada upper
low will support Plains through Great Lakes into eastern Canada
surface low development next week. By day 6 Wednesday there has
been noticeable convergence among a majority of dynamical
models/means plus ML models toward low pressure reaching eastern
Ontario/western Quebec. Note that there is still significant
potential for detail adjustments given the influence of three
separate features.

The ECMWF/GFS and the ensemble means have developed a reasonable
cluster for the upper low expected to reach the Northwest by mid-
late week, with typical scatter for timing and longitude. The 12Z
UKMET/CMC disagreed with this scenario, instead having more sheared
flow reinforcing the mean trough, but the new 00Z runs look more
like the GFS/ECMWF cluster. The ML models offer general support for
this evolution as well.

Guidance comparisons among the 12Z/18Z guidance led to starting
with an operational model composite early in the period followed by
transitioning out the less favored CMC/UKMET after early Tuesday
and then trending to an even mix of GFS/ECMWF and their ensemble
means by the end of the forecast Thursday.


...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
wave should lift a stalling front northward as a warm front late in
the weekend, 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. Compared to 24 hours ago, models
at least show better agreement in the development of some
convection during the current Day 4 period, even with continued
scatter of details. The Day 5 period looks to be more favorable for
potential repeat activity/training in light of slower frontal
motion and persistence of favorable instability/moisture anomalies.
For now both days depict a Marginal Risk area based on the best
combination of guidance parameters, with threats on Day 5 depending
somewhat on how Day 4 transpires. The Day 4 risk area extends from
eastern Kansas/Nebraska into southern Minnesota while the Day 5
area broadens eastward somewhat. Meanwhile the system lingering
near the East Coast could still produce some isolated moderate to
heavy rainfall into Sunday but without enough of a signal to
warrant a risk area. 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 into the eastern and southern U.S. It will take
additional time to resolve most favored areas of heaviest rainfall
in the Tuesday-Thursday time frame.

Initial upper troughing over the West 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. Approach/arrival
of an 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.

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




$$