833
FXUS02 KWNH 300719
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
318 AM EDT Tue Apr 30 2024

Valid 12Z Fri May 03 2024 - 12Z Tue May 07 2024

...Overview...

For late this week, lifting upper troughing and a couple of
surface cold fronts moving east by Friday should push rain chances
across the east-central and eastern U.S., but a subtropical jet and
the western side of the front stalling will lead to additional
threats for heavy rainfall in the south-central U.S. this weekend
as well. Meanwhile, the West is likely to see some precipitation
including some higher elevation snow during the period, but
confidence in the details is still lower than desired given ample
spread in the model guidance.


...Guidance/Predictability Assessment...

As the medium range period begins Friday, a consolidating upper
low should be located near the U.S./Canada border with a surface
low nearby over northern Minnesota or so, while upper ridging is
atop much of the East. Model guidance is within reasonable
agreement with these features though with some detail differences.
The bigger model diagnostics challenge is with an upper low
dropping southeast through the eastern Pacific coming toward the
West Coast this weekend. Deterministic models and ensemble
members/means and AI/machine learning (ML) models vary with the
positioning of this low. Even early in the medium range time frame,
GFS runs have persistently been on the southwestern side of the
spread, which then leads to the upper low farther south into
California and the Southwest as it translates east early next week.
Meanwhile, ECMWF runs do not dig the trough as far south, taking
it across the Northwest over the weekend and quickly into the
northern Plains. The 12Z EC ensemble mean was similarly shallow and
fast with the trough. The 12Z ML models were generally in between
these southwest and northeast extremes with the feature, though
there were some variations. The CMC and UKMET have been in between,
though the 12Z UKMET seemed to be a fast outlier. The incoming 00Z
models seem to keep the same sorts of biases as the 12/18Z cycle.

In going for a middle ground solution for the western low, which
seems the most realistic result, using a blend combining the
faster/shallower ECMWF with the slower/deeper GEFS mean (not quite
as slow/deep as the deterministic GFS runs) was favored for the WPC
model blend. With the preference for an intermediate solution, did
not favor the GFS-type QPF, so backed away from the
precipitation`s southern extent into California and the central
Great Basin by combining the NBM with the ECMWF/CMC. The GFS/GEFS
mean/ensemble bias corrected solutions were farther south than
preferred.


...Weather/Hazards Highlights...

Rain chances are forecast from the south-central U.S.
northeastward into the Lower Mississippi, Tennessee, and Ohio
Valleys on Friday ahead of a frontal boundary. While the threat for
heavy rainfall to cause flash flooding is certainly lower than
earlier in the week because the best upper-level support is
diminishing, a Marginal Risk is in place for the Day 4/Friday ERO
for isolated flooding issues. The Marginal was expanded westward
from the previous issuance into Texas because of the threat for
thunderstorms to develop ahead of the dryline that could have high
rain rates and potentially move slowly. By Saturday, the
subtropical jet gets renewed across the southern Plains and more
focused convection is possible in a moist and unstable environment.
A Marginal Risk will serve as a starting point for the Day
5/Saturday ERO centered in Oklahoma and northern Texas and
surrounding states. Many model solutions have a focused area of
heavy rain near/along the Red River within this, but some models
like the 00Z ECMWF hold off heavier rain until Sunday, so did not
include any embedded Slight Risk for now but that is still a
possibility in future cycles. Many areas in the south-central U.S.
will have quite wet antecedent conditions because of recent and
upcoming short range events, so this will be a factor for flooding
as well. Additionally on Saturday, a tongue of instability is
likely to funnel into the southern/central Appalachians and into
the Piedmont between the synoptic cold front and backdoor front in
the East as a shortwave aloft moves through. Some heavy rain is
likely for these areas so a Marginal Risk for flash flooding is
introduced. Rain and thunderstorms should continue in the central
and eastern U.S. on Sunday.

A round of light to moderate precipitation including higher
elevation snow in Wyoming to north-central Colorado is possible on
Friday. Then the Pacific upper low will provide support to spread
precipitation into the Northwest and steadily eastward into the
weekend and early next week. How much precipitation reaches the
southern side in California and the Great Basin will be dependent
on the upper low/trough. But the Pacific Northwest into the
northern Rockies should see lower elevation rain and higher
elevation snow, with precipitation reaching the northern Plains by
Monday.

Warmer than normal temperatures by 10-15 degrees are forecast for
the Ohio Valley to Great Lakes and Appalachians on Friday. Over the
weekend a backdoor front may cool the Mid-Atlantic to a few degrees
below normal for highs, while the east-central U.S. stays above
normal but less so than on Friday. Farther west, the northern
Rockies and Plains can expect cooler than normal temperatures
through late week, but the Intermountain West to that area should
warm up over the weekend to above normal. Warm temperatures should
spread across much of the eastern two-thirds of the country next
week. 90s are forecast to become more widespread across the
southern tier, with temperatures over 100F into southwest Texas. On
the other hand, the West Coast states are likely to see cooler than
normal temperatures early next week, but with the details uncertain
because of model spread with the trough aloft.


Tate


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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