Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
129 AM EST Mon Jan 11 2021
Valid 12Z Thu Jan 14 2021 - 12Z Mon Jan 18 2021
...Overview...
Amplified mean upper ridging over the West Coast to start the
period (Thursday) will promote a deepening upper trough over the
eastern half of the country late this week. Surface low pressure
associated with this trough may be fairly deep, tracking into the
Upper Great Lakes and Northeast, pushing a cold front into the
East by Friday. This will spread an area of precipitation (more
snow than rain) into the Great Lakes/Northeast and vicinity late
this week into the weekend. The upper trough should lift away from
the East this coming weekend as another Pacific shortwave comes
into the West and amplifies as it moves east through the Rockies
and the Central U.S. into early next week.
...Guidance/Predictability Assessment...
The latest models and ensembles are agreeable and consistent with
the large scale pattern evolution, but are showing some spread and
recent refinements for specifics. There is a fairly strong
consensus now on evolution of Great Lakes low pressure early in
the period. The past day or so of model runs have been showing a
stronger signal for an upper low to close off somewhere near the
Great Lakes, though placement remains a question. The most recent
runs of the UKMET/ECMWF are south of the GFS/CMC/ensemble mean
consensus, however there has been a southward trend in most of the
guidance with this system. Despite these differences, a general
model compromise actually serves well for this system which would
bring a nice deep surface low into the Upper Great Lakes on Friday
and into either far southern Canada or New England by Saturday.
Once the trailing front clears the East Coast, some recent model
runs of the ECMWF and CMC show potential for wave development in
response to sharper shortwave energy in the base of the trough,
while the GFS is more rounded with the upper trough and cleanest
with its frontal passage. Given the lack of confidence in
shortwave specifics, a preference towards minimizing a defined
wave near the East Coast is preferred until there is a more
coherent signal.
The next Pacific shortwave should arrive into the West around
Friday-Saturday, amplifying as it moves across the Rockies and
into the Central U.S. next weekend. The biggest forecast concern
with this is there are indications from some models (most notably
the GFS and CMC) for separated southern stream energy tracking
into the Southern Plains/Gulf Coast states. This would support a
well defined surface low in the northern Gulf of Mexico by day 7
(Monday). The details at this point in time remains highly
uncertain, but recent runs of the GFS are mostly consistent in
showing this. The ECMWF is more suppressed/less enthusiastic about
this scenario, as are the ensemble means which is expected.
Therefore, at this time, prefer a blend closer to that of the
ensemble means with smaller contributions from the deterministic
GFS and ECMWF. This shows at least a surface low/front in the Gulf
on Day 7, but not nearly as defined as the GFS or Canadian runs
would indicate.
...Weather/Threats Highlights...
Leading closed low energy into the Great Lakes would support a
combination of synoptic and lake effect/enhanced snows from the
Great Lakes into the northern/central Appalachians, with
precipitation also extending into parts of the Northeast. Expect
rain in the southern part of the moisture shield along the leading
cold front into the East as well. Some rainfall is also possible
over the Southeast/Florida late this week, though still
considerable spread for coverage and amounts. In the West, the
next shortwave to arrive should bring another round of
precipitation over the Northwest, though less extreme than the
preceding atmospheric river setup now solidly in the short range
period. As this shortwave deepens over the Rockies/Plains this
weekend, precipitation may begin to expand into parts of the Gulf
Coast states as well.
Above normal temperatures should be fairly widespread across much
of the country (with the exception of the Southeast and the
Rockies) to start the period, with the greatest anomalies of
+10-15F from the northern Plains to the Northeast Thursday and
Friday. Amplification of the shortwave reaching the West will
suppress the warmth southward by next weekend, while the upper
trough/front reaching the East late this week will moderate
temperatures back towards or modestly below normal.
Santorelli
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook 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