Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
143 PM EDT Sat Apr 20 2024

Valid 12Z Tue Apr 23 2024 - 12Z Sat Apr 27 2024


...General Overview...

The forecast period begins on Tuesday with an amplifying upper
trough over the Great Lakes and then over the Northeast U.S., and
this will propel a cold front across much of the Eastern U.S. for
the middle of the week. Surface high pressure then builds in across
the Midwest and then across the East to close out the week along
with cooler temperatures. Meanwhile, a synoptic scale trough
develops across the Western U.S. by Friday and into Saturday with a
couple of shortwaves ejecting out across the Plains, and
supporting surface cyclogenesis across the western High Plains.
This will likely result in developing showers and storms for the
Central U.S., with some heavy rain and strong storms possible by
the end of the week.

...Guidance/Predictability Assessment...

The two main forecast challenges are centered on the approach of
two shortwaves in the northern stream, potentially phasing and
closing off in the Nortehast by the middle/late portions of next
week. The GFS and its ensemble mean continue to support a
stronger/more phased system that features a deep closed low over
New York by 00Z Thur while other operational guidance like the CMC
offer a less phased, quicker, split system that pushes through more
subtly into Wed/Thur. The EC AIFS ML model tends to lean toward the
more progressive solution and lines up well with the other non-GFS
solutions.

The other area of concern is across the West with the next puece of
shortwave energy moving into SoCal, Desert Southwest and eventually
ejecting out into the Plains by D6-7. The 00Z/06Z GFS (and now
the 12Z run too) offer a much faster solution (and somewhat
stronger) compared to the non-NCEP guidance (and EC AIFS ML model).
For the WPC model blend preferences and subsequent grids, the
preference was for the slower system and incorporating more of the
ECMWF compared to the GFS.


...Weather/Hazards Highlights...

Upper trough and frontal passages through the Midwest/Great Lakes
and then the Northeast will be accompanied by some widespread light
to moderate rains early-mid next week. Given the overall
progressive nature of this storm system, and the lack of
appreciable instability and QPF most places in this period, no
flash flood threat areas are currently warranted for the WPC
Excessive Rainfall Outlook for Tuesday. On Wednesday, warm air
advection across Texas and Oklahoma will bring a return to
scattered showers and storms across this region, with some of the
model guidance suggesting the potential for some local one inch
rainfall totals. However, the guidance is not locked in on the
location of these scattered QPF maxima, and therefore it seems
prudent to hold off on any risk areas for the Day 5 outlook on
Wednesday until a better model signal develops for placement. More
widespread showers and storms are then expected to develop across
the Plains and Midwest states into later next week as moisture and
instability increase with surface low pressure system development,
and the potential exists for some strong storms ahead of the
dryline in the Southern Plains, so this will continue to be
monitored.

Initial upper level ridging out West should support above normal
temperatures through about Wednesday, and then a return to near or
slightly below normal levels for much of the West Coast and
Intermountain West starting Thursday. Warmer temperatures and
increasing humidity are forecast for the south-central U.S. by
next Friday and beyond as the upper ridge axis builds back, and
southerly flow from the western Gulf advects moisture northward
ahead of a developing surface low. Highs could get over 100 degrees
for portions of southern Texas by Saturday.

Hamrick/Taylor


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