WPC Medium Range Archive
Medium Range Products Valid On:
(Day 7 PMDEPD: Valid 03/17/2019 to 03/21/2019)
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
408 PM EDT Thu Mar 14 2019
Valid 12Z Sun Mar 17 2019 - 12Z Thu Mar 21 2019
Model and ensemble guidance maintains good agreement/continuity in
showing the mean ridge-trough pattern over North America drifting
slowly eastward with time. As has been the case in recent days,
there is still some question as to the specifics of smaller scale
shortwaves within/around the western mean ridge and exact
character of the eastern trough. Meanwhile there is steadily
increasing potential that an upper trough approaching the West
Coast during the first half of next week will close off an upper
low that reaches California around late Wed-Thu. The overall
pattern will be fairly dry across the Lower 48 aside from the
possibility of heavy rainfall over the Florida Peninsula next week
due to a wavy front near the Keys/southern stream shortwave
energy, and moisture moving into the West with the upper low.
An operational model blend (06Z GFS/00Z ECMWF and some aspects of
the 00Z UKMET/CMC) provides a reasonable starting early in the
period. Then increasing 06Z GEFS/00Z ECMWF ensemble mean weight
toward half along with the GFS/ECMWF (and CMC near the West Coast)
helps to smooth out the increasingly uncertain shortwave details
and yields a forecast close to continuity.
Model differences and run to run variability persist for the
evolution of energy initially coming through/around the western
North America ridge and then feeding into the eastern mean trough.
Thus far whichever solutions have deposited the most energy
over/near the northern Rockies/Interior West by early next week
have not held up in successive runs, which in this cycle favors
leaning away from the 00Z GFS/06Z FV3 GFS. Then beyond this issue
there is still a fair degree of spread for the shortwave energy
over the central/eastern states. The 00Z UKMET/CMC are in the
western part of the model/ensemble envelope, with some similarity
to a couple earlier ECMWF runs. On the other hand the 00Z ECMWF
eventually strays to the faster side of the spread. At the moment
GFS runs are closest to the ensemble means. The
spread/variability favor maintaining an intermediate solution for
the deterministic manual forecast, but with the understanding that
the potential still exists for a slightly farther westward trough
and more sensible weather effects over the East. Farther south
there is a reasonable signal that southern stream energy help
generate a frontal wave near the Florida Keys with a track
continuing northeastward but guidance is still attempting to
resolve precise path and timing. A compromise appears best for
this feature as well.
Over the past day the ensemble means have trended deeper and more
closed with the upper trough energy heading toward California by
the middle of next week. This represents a trend toward recent
GFS/ECMWF runs which were earlier to depict a closed system. A
northward adjustment in the ECMWF over the past 1-2 days could be
a tad overdone by late day 7 Thu but otherwise brings it into
reasonable agreement with the majority cluster. The 06Z FV3 GFS
becomes faster than consensus late in the period. GEFS means
become slowest late in the period--plausible in light of multi-day
trends but possibly a little slow based on overall progression of
southern stream flow.
...Weather Highlights and Hazards...
The Florida Peninsula--more likely the southern half--will have
the best potential to see the highest rainfall totals during the
period. A wavy front stalled near/south of the Keys and southern
stream energy aloft will promote a period of unsettled weather
from the weekend into next week, with the best rainfall
enhancement occurring as a more defined frontal wave develops
after Mon. Moisture extending to the northeast from Florida
should remain offshore but it will be close enough to the
southeastern coast that only a slight change in guidance would
lead to more rainfall over that region. Over the rest of the U.S.
east of the Rockies there may be one or more areas of
precipitation with shortwave energy aloft and associated surface
reflections. Any such precip is most likely to be light/scattered
and confidence remains low in specifics. Meanwhile the system
drifting toward California should start to bring moisture into the
state Tue night-Wed with precipitation spreading farther into the
West through Thu. There may be localized areas of focused
activity but location of highest precip totals will be sensitive
to exact track/evolution of the system.
The large scale pattern evolution will tend to favor an expanding
area of plus 5-15F temperature anomalies over the West during the
period, though max temps should cool down over/near California
with arrival of the Wed-Thu system. Upper troughing over the East
will keep that part of the country below normal by up to 5-10F.
Expect chilly readings up to 10-20F below normal over parts of
Texas and the extreme southern Rockies Sun-Mon with gradual
moderation toward normal thereafter. The northern Plains should
also see a warming trend during the period.
- Heavy rain across portions of the Southeast, Tue-Wed, Mar 19-Mar
- Flooding possible across portions of the Central Plains, the
Northern Plains, the Tennessee Valley, the Great Lakes, the Middle
Mississippi Valley, the Upper Mississippi Valley, the Southern
Plains, and the Ohio Valley.
- Flooding occurring or imminent across portions of the Central
Plains, the Middle Mississippi Valley, the Lower Mississippi
Valley, the Ohio Valley, and the Tennessee Valley.
- Flooding likely across portions of the Central Plains, the
Middle Mississippi Valley, the Great Lakes, the Upper Mississippi
Valley, and the Northern Plains.
- Much below normal temperatures across portions of the Southern
Plains, Sun-Mon, Mar 17-Mar 18.
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at: