Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Thu Apr 04 2019
Valid 12Z Sun Apr 07 2019 - 12Z Thu Apr 11 2019
...Overview...
Ongoing progression of Pacific features into and across the Lower
48 will bring active weather to a decent percentage of the country
for one or more days of the forecast period. The southwestern
U.S./southern Rockies appear to be the one area that should have
fairly dry weather. A leading upper trough over the central U.S.
on Sun and its associated surface system will spread a broad area
of precipitation across the eastern half of the country Sun-Tue
with some areas of heavy rainfall possible. During that same time
frame expect a strong jet aimed at the northwestern states to
bring areas of enhanced precipitation to the northern half of the
West Coast into the northern Rockies. An upper trough forecast to
reach the West Coast around early Tue and continue inland should
then push the best focus for precipitation farther east into the
northern-central Rockies and then portions of the
Plains/Mississippi Valley and eventually Ohio Valley and/or Great
Lakes. This trough may support a fairly strong central U.S. storm
system by Wed-Thu. Trailing trough energy reaching the
northeastern Pacific late in the period may approach the West
Coast by next Thu.
...Guidance Assessment/Preferences...
Not a lot has been resolved yet for the overall upper trough
progressing eastward from the Plains. There is still good
agreement on its existence and general timing (especially in the
means) but individual model runs and ensemble members still vary
widely with respect to how energy is distributed within the
trough. Over the past day or so there is been a moderate signal
toward at least a period of separation between flow crossing the
southeast quadrant of the country and that crossing the northeast
quadrant. The southern part of the trough would support a frontal
wave that could provide localized focus for heavy rainfall.
Different variations of stream interaction persist as well.
Continue to prefer a model/mean blend to reflect the most
agreeable aspects of the forecast, in particular low pressure
crossing the Great Lakes/Northeast, while offering a more
conservative approach where greater spread exists.
Fast Pacific flow crossing the Northwest Sun-Mon will likely
contain multiple embedded impulses whose small scale and fast
motion will make them difficult to resolve until the short range
time frame. For the upper trough reaching the West Coast around
early Tue, GFS/GEFS runs through the 18Z cycle were still a bit
slower than than the ECMWF/ECMWF mean. For the 12-18Z cycles the
full array of guidance was not quite as sharply clustered as in
the past couple days when there was a more pronounced majority in
favor of faster ECMWF timing. This led to a somewhat more even
blend for some aspects of evolution, but ultimately the ejecting
trough and corresponding surface low end up closer to the
ECMWF/ECMWF mean late in the period. The 18Z GFS was similar as
of early Thu. New 00Z guidance thus far, including the GFS, is
providing a boost in confidence toward the progressive scenario.
At the same time the 00Z GFS/CMC have trended much stronger with
Plains/Mississippi Valley low pressure Wed-Thu.
Farther upstream models/spaghetti plots still depict a lot of
spread for northern into northeastern Pacific details into Wed.
In spite of this spread through Wed there is actually some
trending toward better large scale agreement by Thu with a trough
evolving along or offshore the northern half of the West Coast.
Combining forecast considerations across the Lower 48, the updated
forecast started with a blend primarily emphasizing operational
guidance early but 50 percent means (12Z ECMWF mean and lesser 18Z
GEFS/12Z NAEFS weight) already by day 5 Tue. The rest of the
forecast tilted a little more toward the means but kept some
attributes of the 18Z GFS/12Z ECMWF to enhance ensemble mean
detail.
...Weather Highlights/Hazards...
Strong flow aloft across the Northwest will promote areas of
terrain-enhanced rain and higher elevation snow from northern
California and the Pacific Northwest into northern Rockies from
Sun into early next week. Current guidance suggests best
potential for heaviest activity will be centered along the West
Coast near the California/Oregon border and the Oregon Cascades.
By Tue the upper troughing moving into the West will begin to
shift the best moisture focus eastward into the Great Basin and
northern-central Rockies. Continued progression of this system
should spread precipitation across the north-central Plains,
Mid-Upper Mississippi Valley, and Ohio Valley and/or Great Lakes
by Wed-Thu. Some activity may be heavy and areas with ongoing
flooding concerns will need to monitor this system. A period of
low level upslope flow may add to totals over the
eastward/northward facing slopes of the Rockies. In addition
stronger trends for Wed-Thu low pressure in latest guidance raise
the potential for strong winds to be an issue over some areas.
Finally, if the system is strong enough there may be some snow in
the northern part of the precip shield to the east of the Rockies.
The eastern half of the country will see a broad area of
precipitation with the system progressing eastward from the Plains
Sun onward. Ample flow of moisture from the Gulf will promote
areas of heavy rainfall. Best potential for highest totals will
be over/near the Lower Mississippi Valley and central Gulf Coast
region and southern Appalachians. Convection may be strong to
severe over southern areas. Check SPC outlooks as specifics
become better resolved over the coming days. Significant rainfall
is also possible from the Ohio Valley into northern
Mid-Atlantic/Northeast. There are still significant detail
uncertainties with this system so confidence in location/magnitude
of heaviest rainfall remains somewhat lower than desired. Snow
should be confined mostly to New England.
The warm sector of the central-eastern U.S. system during the
first half of the period will contain above to much above normal
temperatures. Morning lows will be particularly anomalous with
many locations 15-25F above normal Sun into Tue and some daily
record warm low values may be challenged. The upstream system
will bring a brief period of warmth across the Southwest into the
southern Plans/Southeast over the course of the period, and at the
same time spread below normal readings from the Northwest into the
Plains during Tue-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