Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1111 AM EDT Wed Sep 25 2019
Valid 12Z Sat Sep 28 2019 - 12Z Wed Oct 02 2019
...Much below normal temperatures expected in the West with winter
weather possible for the northern Rockies...
...Much above normal temperatures expected in the eastern third of
the nation...
...A couple potential rounds of heavy rain expected across the
central U.S...
...Pattern Overview and Guidance/Uncertainty Assessment...
Guidance continues to show a highly amplified western
trough/eastern ridge pattern whose large scale typically yields
fairly high predictability. The relative agreement of
latest/recent GFS-ECMWF runs and their respective means supports
this idea. Through much of the period the CMC agreed as well, so
the updated forecast reflected a blend of the 06Z GFS and 00Z
ECMWF/CMC. What differences existed were well within typical
error ranges for the forecast time frame. Even to day 7 Wed the
GFS/ECMWF were sufficiently close to their respective means that a
model/mean blend represented consensus well. Late in the period
the greatest uncertainty involved exactly how approaching Pacific
flow (south of a trough/upper low reaching the northeastern
Pacific) will begin to eject the western trough.
As for some forecast details, the latest guidance cluster is
consistent with the idea of the western trough containing a
Northwest upper low that may not move much between Sat and Mon or
early Tue. At the same time eastern ridge aloft amplifies with
the anchoring upper high retrograding to over or just east of the
Lower Mississippi Valley by early Mon. Shortwave energy ejecting
from the western trough should support a surface wave that emerges
from the West and tracks across the northern Plains/Upper Great
Lakes around Sun-Mon before continuing into Canada. By next
Tue-Wed the front trailing from this wave and extending to and
beyond the central High Plains may decelerate for a time as it
becomes nearly parallel to flow aloft.
...Weather Highlights/Threats...
Guidance continues to highlight heavy precipitation potential over
locations from the northern Rockies/High Plains into parts of the
Great Basin. Expected slow movement of the upper low center over
the interior Northwest should lead to a period of enhanced precip
over terrain favored by southwesterly flow aloft. Meanwhile
eastern slopes of the northern Rockies will see a period of low
level upslope flow. Heaviest snow should fall over western
Montana while some lower elevations to the east may see meaningful
snow as well. Northern California and Pacific northwest rain/high
elevation snow still looks to be lighter and more scattered.
Areas from the Great Basin into the central Rockies may see a
period of relatively stronger winds.
Moisture will spread across the northern Plains as well as the
Midwest/Upper Mississippi Valley/western Great Lakes as low
pressure tracks across the northern tier, with areas of locally
heavy rainfall possible. Then another episode of heavy rainfall
will be possible from the central Plains northeast into the Great
Lakes Tue-Wed as the front trailing from the aforementioned wave
potentially slows down and becomes aligned with flow aloft. This
latter event has the potential to produce heavier totals due to
repeat activity/training, plus possible input of moisture
originating from the tropical eastern Pacific--which on the way
toward the region may also play a part in promoting pockets of
enhanced rainfall over or near the southern High Plains.
Easterly flow around the southern side of the strong eastern ridge
aloft should still lead to a slow increase of moisture over
Florida. The most likely scenario would have Tropical Storm Karen
eventually being steered by this flow, though multi-day trends
have been considerably slower (reaching near the Bahamas by around
day 7 for the past 2-3 days) and the full range of guidance spans
a wide envelope. Monitor NHC forecasts for the latest information
regarding Karen.
Forecasts continue to show a broad area of the western U.S. into
northern Plains with highs 10-25F below normal during most of the
period--and locally 25-35F below normal over some locations in
Montana. Morning lows will tend to be 5-15F below normal from Sun
onward, again locally colder in parts of Montana. Eastern areas
into the Midwest and central/southern Plains will see much above
normal temperatures with anomalies for highs generally plus
10-20F, and some lows possibly 20-25F above normal during the
first half of next week from the central Plains into Great Lakes.
Expect numerous daily record highs/warm lows during the period.
Rausch/Ryan
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