Extended Forecast Discussion...Correction
NWS Weather Prediction Center College Park MD
415 PM EST Sat Mar 02 2019
Correction due to typos in the second section
Valid 12Z Tue Mar 05 2019 - 12Z Sat Mar 09 2019
...Overview...
The flow across the CONUS will begin the medium range in a rather
amplified setup, and then gradually trend more zonal and
progressive by the mid to latter portion of the forecast period.
Shortwave energy ejecting from a mean upper trough off the West
Coast will cross the country every couple days. Meanwhile, a broad
upper trough initially in place across the Great Lakes/Northeast
should gradually lift out by the middle to end of the next work
week.
...Model Guidance Evaluation and Preferences...
Models/ensembles continue to show good clustering with respect to
an energetic upper shortwave and low pressure system approaching
the West Coast Tue night-Wed, such that a multi-model consensus
forecast was preferred. As this feature moves into the central
Rockies by Thu, the same model differences as yesterday continue
to plague this feature. The GFS continues to keep a more amplified
shortwave across the Rockies and into the central U.S., while the
consensus of other guidance continues to weaken the wave as it
encounters broad weakly anticyclonic upper flow. The differences
increase further by Thu night-Fri with the GFS spinning up a deep
cyclone across the central Plains/Midwest, while the consensus of
other guidance keep a weaker and more suppressed surface low
across the southern Plains. Ensemble means continue to provide
little in the way of support for the GFS solution and it was thus
rejected starting Thu night-Fri. Flow in the northeast Pacific
becomes a bit more chaotic by late in the forecast period, with
the ECMWF/CMC showing some degree of shortwave energy undercutting
the North Pacific/Gulf of Alaska ridge, and the GFS keeping a
stronger ridge intact. There is some consensus, however, that an
additional shortwave should move into the western U.S. on Fri and
then reach the central Rockies by Sat. The ECMWF and CMC solutions
showed the best agreement with the ensemble consensus on this
feature.
Given these considerations, the forecast was initially based on a
multi-model deterministic blend (00Z ECMWF/UKMET/CMC and 06Z GFS)
during days 3-5 (Tue-Thu). For days 6-7 (Fri-Sat), the forecast
was shifted to heavier ensemble mean (ECENS and NAEFS) weighting,
with some continued emphasis on the 00Z ECMWF/CMC solutions as
well.
...Sensible Weather and Potential Hazards...
Cold temperatures will continue to make headlines at the beginning
of this forecast period, with a strong arctic surface high in
place across the central/eastern U.S. High temperatures on Tuesday
of 20 to 35 degrees below average are expected across much of the
Plains eastward across the Midwest and Ohio Valley. This equates
to widespread subzero and single digit low temperatures, and highs
only in the teens and low 20s for many of those areas.
Temperatures will only slowly moderate through midweek with
temperatures 15 to 30 degrees below normal also extending eastward
towards the Mid-Atlantic and Northeast. The magnitude and coverage
of the cold temperatures will abate some by the end of the week,
but below normal temperatures will continue for many areas through
the end of next week.
Heavy rain and mountain snow are expected to make a return to the
western U.S. by Tuesday night as an atmospheric river event
accompanies a strong Pacific low headed towards California. Heavy
rainfall across the lower elevations of California may lead to
some flooding problems, and heavy snow in the Sierra Nevada will
add to the already extensive snow pack. Significant snow is
forecast for the central and northern Rockies by Wednesday and
Thursday as the moisture plume spreads inland, albeit not to the
same degree as California. Rain is expected to become become
widespread from portions of the lower/mid-Mississippi Valley to
the Tennessee Valley by Fri into Sat as a warm front sets up ahead
of a developing central U.S. low pressure system.
Ryan/Hamrick
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