Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
356 PM EDT Thu Sep 16 2021

Valid 00Z Fri Sep 17 2021 - 00Z Sun Sep 19 2021

...Post-Tropical Cyclone Nicholas will continue to bring a threat of
locally heavy rain and flash flooding to portions of the Gulf Coast states
over the next couple of days...

...Severe thunderstorms will be possible late this afternoon and this
evening across Minnesota into southeast South Dakota and parts of
Nebraska...

...First atmospheric river of the season will bring very beneficial and
welcomed rains to drought and wildfire-stricken areas of the Pacific
Northwest going into the weekend...

...Very warm, windy, and dry conditions will maintain a wildfire threat
across many areas of the Intermountain West and the northern High Plains
for the next couple of days...

Post-Tropical Cyclone Nicholas continue to linger over far southern
Louisiana this afternoon and should very slowly move northward over the
next couple of days before then dissipating. The system will continue to
bring areas of locally heavy showers and thunderstorms to portions of the
Gulf Coast states for the next couple of days. This will drive concerns
for some additional flash flooding, and the Weather Prediction Center has
depicted a Slight Risk of excessive rainfall for the central Gulf Coast
region stretching from southeast Louisiana east to the western Florida
Panhandle.

Meanwhile, a cold front currently settling southeast across the central
and northern Plains and adjacent areas of the upper Midwest will bring
scattered thunderstorms late this afternoon and evening across this
region, and there will be a threat for some of these thunderstorms to be
severe. In fact, the Storm Prediction Center has depicted a Slight Risk of
severe weather going into the evening hours across Minnesota, southeast
South Dakota, and parts of Nebraska with concerns for supercell
thunderstorms that will be capable of producing large hail, damaging
winds, and a few tornadoes. As the front moves through the Great Lakes
region on Friday and the Ohio Valley early this weekend, there will be
additional scattered areas of showers and thunderstorms associated with
it, but severe weather is generally not anticipated across these areas at
this time.

A well-defined atmospheric river associated with a strong low pressure
system advancing through the Gulf of Alaska will begin impacting the
Pacific Northwest by early Friday morning and will continue doing so
through the weekend. This is the first atmospheric river of the season and
it is expected to bring locally several inches to the coastal ranges of
Washington and Oregon, and also interior areas of the Cascades. Rain
should advance down into at least northern California including the
northern Sierra-Nevada by late in the weekend. This will be very
beneficial and welcomed rain given the current widespread drought
conditions and ongoing wildfire activity.

This system will move into the Rockies by Sunday and much colder
temperatures will settle in behind the frontal passage across the
Northwest which will yield temperatures of as much as 15 to 20 degrees
below average for this time of the year by the end of the weekend.
Conversely, the temperatures out ahead of the cold front will be quite
warm and this will result in a large area of the central and northern High
Plains seeing temperatures at least 15 to 20 degree above average. The
very warm, windy and dry conditions expected ahead of the cold front will
also yield an elevated threat of wildfire activity from areas of the
Intermountain West east into the northern High Plains, and there are
currently Red Flag Warnings in effect to address the wildfire concerns
over the next couple of days.

OrrisonExtended Forecast Discussion
NWS Weather Prediction Center College Park MD
314 PM EDT Thu Sep 16 2021

Valid 12Z Sun Sep 19 2021 - 12Z Thu Sep 23 2021


...Overview and Guidance Evaluation/Preferences...

Elsewhere across the CONUS, high pressure is forecast to be the
dominant feature for the East Coast through the middle of next
week.

Ridging will remain in place over much of the East and Southeast
during the extended period while an amplified trough tracks from
the Pacific Northwest to the north-central states. In general, the
large scale pattern and its evolution have been in fairly good
agreeance the first couple of days, but the spread in timing and
location increases beyond day 5 particularly for the
trough/surface front near the Northern Plains and Great Lakes
region. This trend has been persistent that past 4 or 5 days. A
majority of the guidance show a flattening trough, therefore a
more progressive system to move trough the Pacific Northwest and
Intermountain West into the Plains. Previous runs of the ECMWF
advertised a slower trough that eventually cuts off over the
Plains; and now, the GFS and CMC solutions are hinting at this
posibility but not for the same time periods. The preferred blend
for this forecast consisted of nearly equal weights for day 3-5 of
the ECWMF, GFS and CMC with increasing percentages of the ensemble
means for day 6 and 7 to account for some of the uncertainty in
the northern Plains and Great Lakes.


...Weather/Hazard Highlights...

Warm, moist air will surge northward from the Gulf of Mexico prior
to the trough and associated deepening low pressure system that
will cross the Intermountain West and enter into the Northern
Plains by early next week. As such, the daily temperatures will be
warmer than average and with the mositure surge there will also be
scattered to numerous showers and thunderstorms. Parts of the Gulf
region may have periods of heavier rainfall. The greatest
temperature departures from normal will be for the northern tier
and Upper Midwest where daily highs 10F to 20F above normal will
be possible.

A significant drop in temperatures can be expected across the
Northwest and northern tier states as a "season-changing" cold
front passes through. Scattered to widespread rain will shift from
the Northwest coastal areas inland toward the Intermountain West.
Some of the highest peaks may have snow as the heights and
temperatures fall. Maximum temperatures of 10F to as much as 20F
below normal will be possible Sunday and Monday across the
Pacific Northwest and Intermountain West. In contrast, the daily
lows across the CONUS are largely forecast to be above normal,
with some significant departures from normal likely Sunday into
early next week across the northern tier and Great Lakes. Lows 15F
to 20F above normal are possible.

Campbell