Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Thu Oct 24 2019
Valid 12Z Sun Oct 27 2019 - 12Z Thu Oct 31 2019
...Well below average temperatures expected for much of the West
next week...
...Overview and Guidance/Uncertainty Assessment...
Strongly amplified pattern out of the eastern Pacific across North
America will dominate next week driven by a +3.5 sigma height
anomaly over southeastern Alaska/Yukon territory of Canada. In
addition, persistent subtropical ridging over the Bahamas (+2
sigma) will favor troughing over the western and central CONUS and
broad southwesterly flow through the East. The models/ensembles
have struggled with the amplitude/depth of the troughing, most
notably the GFS/GEFS. Given the lead-in pattern, recent
verification, and long-standing known model biases, will continue
to favor the slower/deeper/more amplified solutions that include
the 00Z ECMWF/Canadian and ECMWF ensemble mean. This pairs well
with WPC continuity. 00Z UKMET also fell into the slower camp but
was among the slowest 5% of ensembles early next week. Multi-day
trends have shown that the deterministic ECMWF can be too
deep/slow with strong troughing and closed off systems, and the
ECMWF ensemble mean is often most correct at the expense of some
lost detail. By next Wed/Thu, additional height falls out of
western Canada may slow/drag the mean trough axis via surface wave
development along the strong cold front (as shown by the past two
ECMWF solutions) but the Canadian was much more progressive.
Trended toward the ECMWF ensemble mean by the end of the period
which does show modest wave development out of the lower MS Valley
into the Great Lakes as most ECMWF ensemble members held the front
along or just west of the Appalachians by 12Z next Thu.
...Weather Highlights/Threats...
Deep troughing in the West will carry a lead cold front into the
Desert Southwest and southern Plains Sun/Mon with temperatures
10-30F below average in its wake, centered from Montana southward
to the Four Corners region and eastward to the western Plains. On
Tue/Wed, another cold front will bring even colder air to the same
region, taking temperatures down to 20-40F below average over much
of Wyoming and eastern Colorado. This may challenge some daily
record lows or low maxes as temperatures stay in the 10s or 20s.
Modest cooling is likely even into the Great Basin and southern
California to the southern Rockies along with increased winds
(~1036mb high pressure over Wyoming next Wed).
The lead front out of the Plains on Sunday will likely get hung up
across the Great Lakes/Midwest and wait for the second cold front
to merge/overtake it with low pressure likely lifting
northeastward. This could lead to a swath of snow in the colder
air to the northwest of the low out of the central Rockies Sun/Mon
and across the central Plains to the Upper Midwest/western Great
Lakes Tue/Wed. Please consult the latest WPC day 4-7 winter
weather probabilities for highlighted areas. As the frontal
boundary pushes eastward into the northwestern Gulf and Lower MS
Valley Wednesday, increased moisture to its east will help expand
the rainfall areas into the TN Valley and northeastward to the
Ohio Valley and Appalachians by Halloween. Confidence is lower
regarding system timing but a slower front would likely favor a
heavier rainfall potential.
Fracasso
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