Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
242 AM EDT Mon Sep 27 2021

Valid 12Z Thu Sep 30 2021 - 12Z Mon Oct 04 2021

...Overview...

The medium range period begins Thursday with an amplified upper
pattern across the contiguous U.S., as ridging in the east-central
U.S. separates a closed low centered over the Northeast and mean
troughing over the West. Initial western trough energy should
split, with the southern part forming a Southwest upper trough or
low and the rest tracking into the northern tier U.S./southern
Canada. Heading into/through the weekend the pattern should
flatten somewhat as a Pacific shortwave reaching western North
America continues on toward the mean ridge position and heights
over a majority of the West generally rise. Late this week the
Southwest upper low and moist inflow will lead to heavy rainfall
potential over southern portions of the Rockies and Plains.

...Guidance Evaluation/Preferences...

The 12/18Z model guidance continues to consolidate around the idea
of a closed upper low centered over the Northeast U.S. Thursday
with a gradual drift northward Friday and into the weekend. Some
differences remain with the roundness or narrowing of the low as
well as the timing of its movement north, with the 12Z GFS as well
as the GEFS means perhaps northern outliers by Saturday. The 18Z
and new 00Z GFS keep the low farther south. On the other hand, the
12Z UKMET split energy southward toward the Mid-Atlantic region by
Friday into Saturday, but its new 00Z run came in closer to
consensus. Thus leaned toward a 12Z EC/ECens mean/CMC and 18Z GFS
solution for the upper low in the East through the weekend.

In the West, guidance has tended to remain somewhat weaker with
energy and the potential for a closed southern stream low in the
Southwest for the latter part of this week. There is still
potential for a closed low at times, but some guidance
(particularly GFS runs) appear to keep the northern and southern
streams a bit more connected rather than closing off a low. But
overall a multi-model deterministic blend worked for this feature.

Meanwhile, a couple of rounds of shortwave energy in the northern
stream with the western trough continue to have some agreement
issues among the guidance. An initial shortwave could move through
the north-central U.S. and/or south-central Canada for the latter
part of the week, but with track and speed differences. Then with
the second shortwave, there is general agreement for it to move
into the Pacific Northwest on Friday, but with some differences
beyond that time. Initially the major outlier is the 12Z CMC,
which has the potent shortwave spinning southward and closing off
an upper low near northern California by early Saturday, unlike
other guidance, which eventually splits energy across the Pacific
but weaker and farther offshore. Through the weekend and into
early next week, the shortwave differences impact the central
CONUS quite a bit. 12/18/00Z GFS runs have energy from both
shortwaves interacting to produce troughing across the Dakotas and
Midwest by Sunday and even forming a closed low over those areas
by Monday. There is some semblance of a north-central U.S. trough
particularly in the GEFS means, with the 12Z EC ensemble mean
showing some mild evidence for it as well. Thus the forecast
trended in the direction of a trough, but not nearly as strong as
the operational GFS runs, and confidence is quite low by early
next week. The WPC forecast quickly trended toward using the
ensemble means by the latter part of the medium range period.

...Weather and Potential Hazards...

Southern parts of the Rockies and Plains can expect multiple days
of widespread and potentially heavy rainfall for the latter half
of the week, as the upper low in the Southwest provides moist
inflow from the Pacific and the low-level pattern brings moisture
from the Gulf of Mexico. The specifics of rainfall duration and
intensity remain somewhat uncertain given model differences in the
timing and track of the Southwest upper low, but the current
forecast shows 2 of 4 inches of rain with locally higher amounts
in the southern High Plains. Periods of rain are also possible
farther north toward the rest of the Plains and the Midwest in the
vicinity of a couple of frontal boundaries. The Pacific Northwest
should see precipitation lasting through Thursday, with
orographically enhanced amounts in the Olympics and northern
Cascades, and some snow is possible at the highest elevations.
Scattered showers and thunderstorms are likely across southern
Florida as a couple of fronts move through. The rest of the East
should be generally dry underneath high pressure, other than
showers in the Northeast mainly Thursday due to the nearby upper
low, and some possible moisture approaching the Ohio Valley into
the Mid-Atlantic by Sunday.

Warmer than average temperatures are forecast across the
north-central tier of the CONUS through the period, with highs in
the 70s, even 80s in the Midwest underneath upper ridging. On the
other hand, high temperature anomalies of 10-20F below average
should be centered across central/southern portions of the Rockies
and High Plains given the clouds and precipitation over the
region. The West Coast should warm up above normal by the weekend,
while slightly below average temperatures are expected for the
Northeast for as long as the forecast upper low persists over the
region.

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, winter weather outlook probabilities
and heat indices are at:

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/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml