Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
330 AM EDT Fri Sep 20 2024

Valid 12Z Mon Sep 23 2024 - 12Z Fri Sep 27 2024


...Guidance/Predictability Assessment...

The 12 UTC guidance suite was overall in better than average
agreement through early-mid next week for the mid-larger scale
pattern evolution and main systems. However, the 06 UTC GFS/GEFS
introduced additional outllier variance. Opted for a composite of
best clustered guidance from the 12 UTC GFS/ECMWF and NAEFS/ECMWF
ensemble means that seemed to offer a good forecast basis in
conjucture with the National Blend of Models. The majority of blend
weighting was applied to these models in this period, and then to
the ensemble means later next week amid growing uncertainty and in
an effort to best emulate NHC preferences for depiction of possible
Gulf Tropical activity and as per WPC system continuity.

...Weather/Hazards Highlights...

Energy over the West will splits southward over the Rockies, then
southwestward toward the Southwest through early-mid next week.
This may provide a period favorable for enhanced rainfall over the
southern High Plains and southern Rockies Tuesday into Wednesday 
in upper diffluent flow and with frontal proximity. A WPC Marginal
Risk area has been introduced.

Meanwhile farther north, upper trough energy and wavy frontal
systems will be kicked from the Plains to the Midwest/East early
next week along with an associated threat of showers/thunderstorms
with some local runoff issues. WPC Excessive Rainfall Outlook (ERO)
Marginal Risk areas are depicted to shift eastward Day4/Monday and
Day/5 Tuesday across the Great Lakes/Ohio Valley to the northern
Mid-Atlantic. Upper trough reinforcement may support additional
heavy rainfall potential to monitor over the East mid-later next
week in amplifying flow. This occurs as far upstream development
of a potent and slowly progressive upper trough from the northeast
Pacific early next week provokes downstream upper ridge
amplification and warming, with an eastward shift from the NOAM
West Coast to mid-continent next week.

Meanwhile NHC and WPC will continue to monitor possible tropical
development in the Caribbean that may slowly move northward next
week. The ultimate position and southward breath of the upper
trough over the lower 48 will determine extent of northward
lifting. Latest 00 UTC models have trended westward and split.

Farther west, most areas should stay dry under mean ridging. The
Washington coastal ranges/Cascades could be an exception and offer
growing potential over time with subsequent ample Pacific energy
approach. Then in South Florida, showers and storms may focus near
a frontal boundary lingering in a moist environment. These do not
look to be too heavy next week until around late week, when there
may be some surge of tropical moisture northward ahead of any
potential tropical development farther south in the Caribbean that
may slowly make its way north into the Gulf during next week. There
is still considerable uncertainty in the track and timing of any
possible tropical system, so continue to check updated forecasts.

Schichtel


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