Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Thu Sep 26 2019
Valid 12Z Sun Sep 29 2019 - 12Z Thu Oct 03 2019
...Much below normal temperatures over the West/northern Plains
with heavy snow possible in the northern Rockies...
...Much above normal temperatures over the East, extending for a
time as far west as parts of the Plains...
...Heavy rainfall potential from the southern High Plains into
Great Lakes...
...Pattern Overview and Guidance/Uncertainty Assessment...
From Sun into Mon the models and ensemble means remain agreeable
and consistent with the forecast of a very amplified pattern
consisting of a western trough containing an upper low over the
Northwest and eastern ridge whose upper high center reaches near
the Lower Mississippi Valley. The first guidance discrepancy
arises as some combination of ejecting western trough energy and
southern Canada flow propels northern Plains low pressure on Mon
northeast into Canada. After Tue consensus shows a fairly rapid
flattening of the mean pattern as a northeastern Pacific trough
drops southeast toward the West Coast, ejecting the initial
western trough. There has been some spread/variability with how
quickly the initial trough departs (though perhaps a little less
than could be the case in such situations) and with details of
height falls reaching the West Coast by next Thu.
Based on guidance through the 12Z-18Z cycles, an operational model
blend represented consensus well for days 3-4 Sun-Mon. Even into
day 5 Tue a model blend with a modest introduction of the 18Z
GEFS/12Z ECMWF means provided a reasonable compromise for timing
of the northern Plains into Canada surface low. The 12Z ECMWF and
12-18Z GFS runs jumped faster than some previous runs and 12Z-00Z
CMC/UKMET runs and the new 00Z GFS is even faster. Ensemble mean
signals have been mixed as well with the ECMWF mean supporting the
ECMWF/GFS versus the slower CMC/GEFS means. Over a multi-day time
frame the timing has varied and important upper details are small
enough in scale to have relatively lower predictability--favoring
an intermediate approach.
Over the past couple days the operational models and ensembles
have shown fairly significant spread for specific details of the
northeast Pacific trough dropping toward the West Coast. The
primary guidance consideration was to phase out the 12Z ECMWF from
the model/ensemble mean blend after day 6 Wed due to its faster
trough arrival versus most other solutions. The new 00Z
GFS/CMC/GEFS mean maintain support for the trough axis remaining
off the West Coast into Thu.
...Weather Highlights/Threats...
The upper low over the Northwest and strong low level easterly
flow north of surface low pressure over the north-central Rockies
will support continued heavy precipitation over the northern
Rockies through the weekend. The greatest snow totals should be
over higher elevations of western Montana with some snow possibly
extending to somewhat lower elevations to the east or northeast.
The surface low will likely bring areas of locally heavy rainfall
across the extreme northern Plains into the Upper Mississippi
Valley/Upper Great Lakes Sun into Mon.
At the same time flow around the western side of the eastern ridge
aloft should begin to bring rainfall-enhancing moisture from the
tropical eastern Pacific and Mexico into the southern High Plains.
This flow may persist through midweek, leading to multiple
episodes of locally heavy rainfall over the region. The
interaction of this moisture and wavy front trailing from the
northern Plains/Canada low will raise the potential for a heavy
rainfall event from the central Plains northeast into the Great
Lakes (centered over Iowa). The signal for this event has
persisted over the past couple days and the possibility that the
front moves slowly for a day or two would allow for significant
rainfall totals due to training/repeat activity. To a lesser
degree some moisture should extend into New England.
Deep-layer easterly flow will promote an increase in rain/showers
over Florida. The flow may ultimately contain some moisture
associated with Tropical Storm Karen, but Karen itself is
currently forecast to weaken slowly to a tropical depression and
then a post-tropical remnant low in the coming days. Monitor NHC
forecasts for the latest information regarding Karen.
The most extreme temperature anomalies over the lower 48 will
exist from Sun into early next week. The western U.S. into
northern Plains will see highs 10-25F below normal with localized
areas in Montana seeing highs at least 30F below normal. Daily
records for cold highs are most likely on Sun but some are
possible beyond then. The east will see a broad area of plus
10-20F anomalies for highs with numerous daily records. The
northern Plains low pressure early in the week will also bring a
surge of warmth into parts of the central U.S. with morning lows
particularly warm (plus 20F or greater anomalies) from the central
Plains into Great Lakes. As the pattern aloft begins to trend
less amplified expect a gradual moderation of chilly western
U.S./northern Plains temperatures while warm to hot readings
become more confined to the East--with some record highs/warm lows
continuing to be possible into Wed-Thu.
Rausch
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