Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1202 PM EDT Sun Sep 15 2019
Valid 12Z Wed Sep 18 2019 - 12Z Sun Sep 22 2019
...Overview and Guidance Evaluation/Preferences...
The two main weather features through the medium range period are
the trough digging over the Great Basin through Thursday then
ejecting northeast across the northern Great Plains through next
weekend and Humberto. For the western trough, the 00Z/06Z models
and ensembles are in good agreement through Day 4/Thursday before
the 06Z GFS becomes more progressive than the 06Z GEFS mean while
the 00Z ECMWF and 00Z ECENS mean keep good timing into Day
6/Saturday. However, the 00Z GFS remains well aligned with the 00Z
GEFS mean into Day 5. Therefore the 00Z GFS is used in the blend
through Day 5 with a switch to the 06Z GEFS mean for the remainder
of the period. The 00Z UKMET is quicker to eject the trough out
west like the GFS, so a small amount of it was included through
Day 4/the limit of its availability at WPC. There are differences
again on Day 7 with the progression and strength of the next
trough toward the Pacific Northwest with the 00Z ECMWF much more
progressive than both the 00Z and 06Z GEFS. However, the ensemble
means are not that far apart, so a heavy reliance on ECENS and
GEFS means is used for Days 6/7. The 00Z CMC never ejects Humberto
east of Bermuda like the rest of the global models. In fact, the
00Z CMC brings Humberto back toward NC on Day 5, so it was
completely disregarded for this blend.
...Weather Highlights/Threats...
Tropical Storm Humberto continues to be forecast by NHC to be a
hurricane as it shifts east near Bermuda during the midweek.
As the upper trough amplifies from the Pacific Northwest expect
widespread rains (some very high elevation snow) to WA/OR
Wednesday, spreading inland/south to MT/ID and northern NV/CA on
Thursday. As the associated surface low spreads onto the northern
Plains, expect an organized convective focus with locally heavy
downpours shifts to the northern Plains Friday into Saturday
before shifting east to the Great Lakes and central Plains Midwest
next weekend.
Increased moisture inflow up the Plains ahead of this system will
lead to near record high minimum temperatures Wednesday with above
normal temperatures there continuing through the rest of the
period. The upstream pattern meanwhile reloads again over the West
to reach the north-central states again next weekend.
In this pattern expect subtropical moisture out of Mexico on
southwest flow aloft over the Southwest/Four Corners states with
rain chances mainly on Days 4/5. This moisture may fuel support
for aforementioned potential convection downwind into the
north-central U.S. Strong ridging downstream over the east-central
U.S. will favor well-above average temperatures next week with
some near record Deep South heat. Nearby in the western Gulf of
Mexico, a trough/wave associated with an upper low favors a
potentially heavy over coastal Texas midweek and weak steering
aloft may allow this area of rain to persist through late week.
Jackson
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