Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Mon Sep 23 2019
Valid 12Z Thu Sep 26 2019 - 12Z Mon Sep 30 2019
...Tropical Storm Karen over the Caribbean...
...Rain and heavy mountain snows with much below normal
temperatures over the West...
...Heatwave for the South/Southeast U.S...
...Heavy thunderstorms/rain for parts of the central U.S....
...Pattern Overview and Guidance/Uncertainty Assessment...
There is still good overall agreement that a very amplified
western trough/eastern ridge upper pattern will become established
by the weekend and extend into the first part of next week. Much
below normal temperatures and areas of precipitation will
accompany the western trough with the northern Rockies likely to
see the highest totals of rain and higher elevation snow. Farther
east a couple waves emerging from the West--along with energy from
the upper low initially over the extreme Southwest--will increase
potential for heavy rainfall from parts of the Midwest into the
Upper Mississippi Valley/Upper Great Lakes. The eastern ridge
aloft will support a broad area of much above normal temperatures
over the East and into the central/southern Plains with record
highs most likely over and near the Southeast. Meanwhile, Tropical
Storm Karen will require monitoring as the southeastern
U.S./western Atlantic ridge at the surface and aloft may direct
the system toward the Bahamas late in the period.
The WPC medium range product suite was primarily derived from a
composite blend of reasonably well clustered guidance from the 00
UTC GFS/ECMWF/Canadian and 00 UTC GEFS/ECMWF ensemble means. This
maintains good WPC continuity in a pattern with above normal flow
transition predictability.
...Weather Highlights/Threats...
Locations over and near the Bahamas and Florida may see increased
rainfall toward the end of the period, whether just from moist
easterly deep-layer flow or the potential approach of Karen.
Monitor NHC forecasts for the latest information regarding Karen.
Highest wintery precipitation totals over the West should be over
the northern Rockies and possibly extending east into the northern
High Plains and south into parts of the Great Basin. Best
potential for significant snow will be over higher elevations of
western Montana, while snow levels could trend sufficiently low to
reach some valley locations. A period of low level upslope flow
may enhance precipitation as well. Other areas over the West
should see lesser amounts and with greater sensitivity to
specifics of the upper trough. Wind will increase over much of the
interior West as well, centered over the Great Basin eastward into
the central Rockies.
During Fri-Mon expect multiple days with highs 10-25F below normal
over parts of the West and extending into the northern Plains. The
core of most extreme anomalies should cover an area from parts of
Oregon and northeast California/northern Nevada into Montana.
Morning lows will be less extreme due to clouds and precip but
there may be pockets of min temps of at least 10F below normal.
Parts of the East/Southeast will see a protracted heatwave with
plus 10-20F anomalies for highs and lows, with record lows also on
tap from the south-central Plains then into the Midwest later
period.
The overall pattern and specifically two separate waves will
promote heavy rainfall potential over areas from the Midwest into
the Upper Mississippi Valley and Upper Great Lakes. Energy
originating from the compact upper low initially over the extreme
Southwest may contribute to focused activity but with some
uncertainty over timing and extent. The 00 UTC GFS/ECMWF runs do
now offer a more common and less progressive solution that seem
reasonable considering the initial closed nature of the system.
Schichtel
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards 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