Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1200 PM EDT Sat Mar 16 2019
Valid 12Z Tue Mar 19 2019 - 12Z Sat Mar 23 2019
...Overview...
Models and ensembles show a blocking mid/upper-level ridge
initially centered from the Pacific Northwest to western Canada
early next week drifting slowly eastward through the forecast
period. Downstream from this ridge expect a mean upper-level
trough initially across the Great Lakes/Northeast to lift out
before deeper troughing may amplify into New England around next
Fri-Sat. Meanwhile a shortwave over the northern Plains should
progress southeastward through the eastern U.S. early-mid period.
Guidance continues to depict strong North Central Pacific westerly
flow splitting as it approaches the western coast of North
America. Some of this energy will form a system crossing the
southwestern U.S. while the rest flows into the higher latitudes
of Canada. Stronger height falls may approach the
central/northern West Coast by next Fri-Sat as the CONUS portion
of the mean ridge shifts far enough eastward.
...Guidance Assessment/Preferences...
The updated forecast emphasizes the 00Z ECMWF/UKMET/CMC (with a
small 00Z NAEFS mean component) from day 3 Tue into day 5 Thu and
then adjusts to an even model/mean blend consisting of the
ECMWF/CMC and ECMWF/NAEFS means.
This cluster represents the current majority for the trough/upper
low expected to track into the southwestern U.S. and the Rockies
mid-late week while depicting an intermediate timing for the
shortwave dropping southeastward from the northern plains. For
the system coming into the West, latest GFS/GEFS mean runs have
backed off somewhat from yesterday's fast timing but not yet to
the point of joining consensus. That said, the ECMWF has trended
a tad faster over the past day so the slowest solutions (the 00Z
CMC mean in today's case) may not be ideal either. The 06Z FV3
GFS compares much better to the favored blend than the operational
GFS runs.
Forecasts to the southeast of closed highs--such as the one over
southwestern Canada as of early Tue--are notoriously difficult so
it is not exactly surprising to see the great spread and run to
run variability seen thus far for the shortwave energy progressing
from the northern Plains through the East. Guidance trends have
generally been in the direction of UKMET/CMC ideas from the past
couple days so it seems reasonable to account for those runs and
similar ECMWF. This cluster is a good midpoint between the fast
00Z GFS and slow 06Z GFS.
During the latter half of the period the models/means are more
agreeable than average with height falls reaching the West Coast,
favoring a model/mean blend. The 06Z FV3 GFS is quite deep/sharp
versus other guidance for upper troughing that amplifies into the
northeastern U.S. The 06Z GFS may be a little too amplified with
the southwestern part of the trough but other models/means agree
fairly well.
...Weather Highlights and Hazards...
The Pacific system tracking into California should spread rain and
high elevation snow into the Sierra Nevada, Great Basin, and
central/southern Rockies by next Wed-Fri. Some localized
enhancement of precipitation over favored terrain will be
possible. By Fri-Sat expect moisture to increase ahead of the
system, supporting greater coverage/intensity of showers and
thunderstorms over portions of the Southern Plains with locally
heavy rain possible. Also in that time frame the stronger height
falls aloft and a cold front approaching the Pacific Northwest and
northern California may bring a return of rain/mountain snow to
that area.
Expect widespread and potentially heavy rain over the southern
half of the Florida Peninsula on Tue and perhaps into Tue
night-Wed as a wave of low pressure moves along a surface front
stalled south of the state. Some rainfall of mostly
light-moderate intensity will be possible from the central Plains
eastward Tue-Thu in association with an upper shortwave and weak
surface wave/frontal system. By late in the week locations near
the coast of the Carolinas may see a brief period of rainfall as
low pressure develops off the southeastern coast but then tracks
rapidly farther offshore. Northern parts of the East should be
fairly dry except for light late week/early weekend snow over
favored terrain of New England behind a cold front that drops
southeastward from Canada.
A majority of the West should see above normal morning lows during
the period (plus 10-15F anomalies at some locations) while the
system crossing the southern half of the West will keep highs near
to below normal over affected areas after Tue. The southern High
Plains will tend to be on the cool side versus normal during the
period but the northern-central Plains will likely see near to
above normal temperatures as warmth spreads from the West. The
eastern U.S. will be 5-10F below normal Tue-Wed but moderate
toward normal thereafter. A frontal passage could bring cool and
breezy conditions to the Northeast by next Sat though.
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