Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
308 PM EDT Fri Mar 15 2019
Valid 12Z Mon Mar 18 2019 - 12Z Fri Mar 22 2019
...Overview...
A blocking mid/upper-level ridge initially centered from the
Pacific Northwest to western Canada early next week should very
slowly shift eastward through the forecast period. An upstream
trough reaching the northeastern Pacific by the latter half of the
week may help to reinforce/rebuild the western Canada ridge by
Fri. The western North America ridge will promote a mean trough
over the eastern part of the continent. Multiple shortwaves
embedded within the overall trough--less predictable due to their
smaller scale--will make it difficult to resolve some forecast
details. Strong westerly flow across the North Central Pacific
will split as it encounters the ridge, with some shortwave energy
directed north into the higher latitudes of Canada and the rest
separating into the southern stream. By late next week the
persistence of ridging over western Canada may begin to direct
stronger height falls from the North Pacific closer to the Pacific
Northwest.
...Guidance Assessment/Preferences...
Confidence in specifics of the system forecast to approach/reach
California around midweek has decreased over the past day due to a
significant divergence of solutions for the feature itself and
upstream flow that may influence it. GFS runs (the 00Z version in
particular) have trended toward much stronger upstream energy
reaching the Pacific Northwest--leading to a faster and more open
upper system reaching California. On the opposite side of the
spectrum, the 00Z CMC becomes slower than consensus with flow
farther upstream over the Pacific--resulting in an upper low that
becomes an extreme to outlier southwestward solution by Thu-Fri.
Overall prefer the 00Z ECMWF/ECMWF mean/CMC mean and 06Z FV3 GFS
which represent the most consistent forecast relative to prior
runs. Modestly slower adjustment in the 06Z GFS provides a little
added confidence in the favored scenario. Note that by late next
Fri the 06Z FV3 GFS may become a little too aggressive with its
shortwave nearing California.
At the same time the models and individual ensemble members
continue to be quite diverse for the evolution for shortwave
energy reaching the northern Plains and vicinity by Tue and
continuing into the East thereafter. UKMET/CMC runs have been
persistent in depicting a slow and amplified/closed feature while
latest GFS/ECMWF runs are quite open and progressive. Some ECMWF
runs from a couple days ago were slower though. Teleconnections
relative to the positive height anomaly center seen in multi-day
means over western Canada leave the door open to some trough
elongation over the Midwest/Plains but perhaps with greater
progression than the UKMET/CMC scenario. Would recommend a
blended approach for the purposes of a single deterministic
forecast.
Later in the week there are typical differences in central/eastern
Canada flow that may amplify into the Northeast. A model/ensemble
mean blend looks reasonable for this part of the forecast, aside
from the 06Z FV3 GFS that becomes sharper and more amplified than
most other guidance.
Based on preferences over the West in particular, the forecast
places greatest emphasis on the 00Z ECMWF during the first half of
the period with minority input from the 00Z UKMET/CMC and 06Z FV3
GFS. After midweek the forecast reduces CMC/FV3 input while
increasing weight of the 00Z ECMWF mean and NAEFS mean (which
includes the CMC ensembles).
...Weather Highlights and Hazards...
A lingering frontal boundary south of the Florida Peninsula and
one or more embedded waves will support widespread and potentially
heavy rains over the southern half of the Florida Peninsula during
the first half of the week. Farther west, the preferred forecast
for 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. Note that a less likely scenario exists that would
bring more moisture into northern California. Moisture from
upstream flow may reach the Pacific Northwest by next Fri. One or
more areas of mostly light rain/snow may progress across the Great
Lakes/Northeast within cyclonic flow aloft, with some activity
possibly extending farther southwestward along a front.
The most prominent temperature trend next week will be the
development of above normal readings (especially for morning lows)
over the West and progressing/extending into the north-central
Plains. There should be decent coverage of plus 10-20F anomalies
for min temps over the Interior West by Wed-Thu and parts of the
High Plains by Fri. Warmest highs (plus 10F or so anomalies) over
the West will likely be near the West Coast early in the week
while highs from California into the Four Corners states trend
gradually cooler with the arrival of clouds/precipitation. Cool
temperatures over the South/East (10-15F below normal over
southwestern Texas early in the week, generally 5-10F below normal
elsewhere) early in the week should moderate closer to normal by
Thu-Fri.
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