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Valid Monday March 19, 2018 to Friday March 30, 2018
US Hazards Outlook
NWS Climate Prediction Center College Park MD
EDT March 16 2018Synopsis
: A very busy period is in store for
Week-1 across the entire country, before uncertainty grows in the second half
of Week-2 regarding the forecast circulation, limiting confidence in subsequent
hazards. Surface low pressure is forecast to be in the Central Plains at the
beginning of the period, and subsequently track to the east-northeast. A second
region of surface low pressure may develop off the Mid-Atlantic coast in the
wake of the aforementioned system. Mid-level low pressure is forecast to
approach the Pacific coastline by the middle of Week-2, favoring a wet pattern
for the West and development of another surface cyclone downstream of the
Northern Rockies late in Week-1. During Week-2 mid-level low pressure is
forecast over the West and off the east coast, with mid-level high pressure
over the Southern Plains. The latter feature is forecast to slide eastward over
the course of Week-2, with uncertainty regarding the evolution of the pattern
across much of the remainder of the country. Western mainland Alaska is
forecast to be influenced by a region of surface low pressure tracking inland
from the Bering Sea that may help bring a reinforcing shot of cold air across
the state in its wake. Temperatures may moderate across the state by the second
half of Week-2, while fairly dry conditions are otherwise anticipated.
Summary For Monday March 19 - Friday March 23:
- Severe weather across portions of the Southeast,
the Lower Mississippi Valley, the Southern Appalachians, and the Tennessee
Valley, Mon, Mar 19.
- Heavy rain across portions of the Southeast, Mon, Mar 19.
- Heavy snow across portions of the Mid-Atlantic, the Northeast, and the
Central Appalachians, Tue, Mar 20.
- Heavy rain across portions of the Southern Appalachians, the Mid-Atlantic,
and the Central Appalachians, Tue, Mar 20.
- High winds across coastal portions of the Mid-Atlantic and the Northeast,
Tue-Wed, Mar 20-21.
- Heavy rain across portions of California, Wed, Mar 21.
- Heavy snow across portions of the Sierra Nevadas, Wed-Thu, Mar 21-22.
- Heavy snow across portions of the Oregon Cascades and northwestern
California, Thu-Fri, Mar 22-23.
- Heavy snow across portions of the Upper Mississippi Valley and the Northern
Plains, Fri, Mar 23.
- Flooding occurring or imminent across portions of the Middle Mississippi
Valley, the Lower Mississippi Valley, the Great Lakes, the Ohio Valley, and the
- High winds across portions of the Central and Southern High Plains, Mon,
- High winds across portions of the Central Rockies, and the High Plains,
Fri, Mar 23.
- Much below-normal temperatures across portions of the Mid-Atlantic, the
Northeast, and the Great Lakes, Mon-Tue, Mar 19-20.
- High winds across portions of western mainland Alaska, Mon, Mar 19.
- High winds across portions of southern mainland Alaska, Mon-Thu, Mar 19-22.
- Much below-normal temperatures across portions of southeastern mainland
Alaska, Thu-Fri, Mar 22-23.
- Moderate risk of much below normal temperatures for portions of eastern
mainland Alaska, Sat, Mar 24.
- Slight risk of much below-normal temperatures for much of the West,
Sat-Sun, Mar 24-25.
- Slight risk of much below-normal temperatures for portions of the
Northeast, the Great Lakes, the Mid-Atlantic, and the Central Appalachians,
Sat-Sun, Mar 24-25.
- Slight risk of much below-normal temperatures for portions of southeastern
mainland Alaska, Sat-Sun, Mar 24-25.
- Slight risk of much above normal temperatures for portions of South Texas,
Sat-Sun, Mar 24-25.
- Slight risk of heavy precipitation for portions of the Lower and Middle
Mississippi Valley, the Great Lakes, the Ohio Valley, and the Tennessee Valley,
Sat-Mon, Mar 24-26.
- Severe Drought across the Central Plains, the Central Rockies, the Central
Great Basin, the Northern Plains, the Southern Rockies, California, the
Southeast, the Southern Plains, and the Southwest.
By the start of the forecast period a potent surface low is forecast to be
located near the Kansas-Oklahoma border, with attendant cold and warm fronts
extending to the south and east, respectively. Behind the system, downslope
flow across the Central High Plains appears likely to yield high winds
(sustained winds above 35 knots, with possible gusts above 50 knots) on the
19th. The Storm Prediction Center (SPC) also anticipates marginally elevated
fire weather conditions in association with this setup across the Southern
Plains with warm, dry downslope flow and the presence of dry fuels in portions
of West Texas and southeast New Mexico on the 19th. Ahead of the system, ample
low-level flow out of the Gulf of Mexico is anticipated to moisten the warm
sector of this system. With this moisture availability, coincident forcing
mechanisms from the northward-lifting warm front, forecast steep mid-level
lapse rates, and likely sufficient vertical wind shear near the triple point of
the system, there is severe weather possible on the 19th of March across the
Tennessee Valley and Southeast. Forecast low-level hodographs suggest tornadic
supercell potential with substantial low-level helicity, despite isallobaric
considerations typically being the worst modeled component for severe weather.
Upscale storm growth is possible towards the overnight hours with the advancing
cold front, which may shift the primary concern to severe winds. For more
information on the possible severe weather threat with this system please see
the SPC or your local weather forecast office. Widespread heavy rain (exceeding
1 inch in 24 hours) is possible across Alabama, Georgia, and South Carolina on
the 19th with this system, with locally heavy rain possible elsewhere.
As the aforementioned system progresses to the north and east, another
round of precipitation-driven threats is possible for the Mid-Atlantic and
Appalachians through southern New England. Heavy snow (exceeding 6 inches in 24
hours) is possible to the north and west of the major metropolitan areas along
the Interstate-95 Corridor on the 20th, with the likely exception of Boston.
Cold air appears likely to be in place ahead of this event for the entire
region, although much below-normal temperatures of -12 degrees F or greater
appear to be confined to upstate New York and New England on the 19th and 20th.
Closer to the coast, heavy rain becomes possible across the Mid-Atlantic on the
20th. Models are inconsistent in how much to deepen the low when it enters the
North Atlantic, but high winds (exceeding 35 knots sustained or gusting above
50 knots) appear possible for coastal areas from the Mid-Atlantic through New
England on the 20th and 21st. Models also try to spin up a secondary low
pressure system off the Mid-Atlantic coast in the wake of the first system,
which may bring a secondary shot of precipitation to the region.
The West is not left out of the hazardous weather in store during Week-1,
with a late season atmospheric river waiting to come ashore during the period.
Heavy rain (widespread totals of 1-2 inches in 24 hours) is likely for coastal
portions of California between Los Angeles and the Bay area on the 21st.
Further inland, heavy snows are possible across the Sierra Nevadas on the 20th
and 21st where up to a couple feet of snow are possible. The primary
precipitation axis is forecast to shift northward somewhat on the 21st,
bringing the potential for heavy snow to mountainous areas in northwestern
California and the Cascades of Oregon on the 22nd and 23rd with daily totals
above 6 inches likely. Snow levels are likely to begin below mountain pass
levels and rise over the course of the event given the southwesterly flow off
With mid-level troughing shifting into the Western U.S. and bringing the
aforementioned precipitation to points closer to the coast, this setup favors
cyclogenesis in the lee of the Central Rockies. Today's 12Z GFS develops a
surface low over Montana by 0Z on the 23rd that is then forecast to track
eastward. As it does, heavy snow is possible on the north side of the system on
the 23rd across the Dakotas and into Minnesota. On the back side of the system,
high downslope winds are possible across much of the High Plains also on the
23rd. As with the system earlier in the week, adiabatic warming and drying are
likely with this downslope flow that may help raise fire weather concerns
across the High Plains, although SPC outlooks currently do not extend to this
Alaska is forecast to be impacted by a region of surface low pressure
approaching the state from the Bering Sea early in Week-1. This may yield an
area of high winds (exceeding 35 knots) on the 19th for areas surrounding the
Norton Sound. In the wake of this system, Arctic air appears likely to spill
southward across the state. The cold air spilling southward may result in high
winds associated with gap wind events across many southern portions of the
state on the 19th through 22nd that could gust in excess of 80-100 mph. As the
cold air builds across the interior of the state, much below-normal
temperatures are possible from the 21st through 23rd with daily minimum
temperature anomalies in excess of -20 degrees F. For Saturday March 24 - Friday
The forecast 500-hPa circulation for Week-2 features anomalous
ridging over the North Pacific, coupled with anomalous troughing initially over
the Pacific Northwest and off the eastern seaboard straddling an anomalous
ridge over the Southern Plains. Over the course of Week-2, the anomalous ridge
over the Southern Plains is forecast to shift towards the Southeast, with
heights falling across the Rockies as some degree of troughing shifts eastward.
The ECMWF ensemble is more amplified with this trough relative to the GEFS,
while the GEPS is similar in amplitude but less progressive and maintains the
feature over the West throughout Week-2. The GEFS and Canadian ensembles are
more amplified with ridging in the South during Week-2, whereas the ECMWF
ensembles approach climatological height values by the second half of Week-2.
Given the general shift from a higher amplitude pattern and greater consensus
among model guidance during the first half of Week-2, all currently anticipated
hazards during this period are forecast in the early part of the week as
confidence wanes substantially beyond day 10.
Anomalously cold weather is forecast to linger from Week-1 into the early
part of Week-2 across eastern mainland Alaska. The GEFS forecasts ensemble mean
temperature anomalies of -20 degrees F or greater on the 24th, suggesting a
moderate risk being warranted. The GEFS reforecast guidance also suggests a
better than 40% chance of this region being below the 15th climatological
percentile of minimum temperature on the 24th, and better than 20% chance of
being in this bottom portion of the daily low temperature distribution on the
24th and 25th, leading to a slight risk of much below-normal temperatures.
Beyond this period signals wane both in terms of the forecast anomalies and
reforecast guidance values relative to climatology.
With the anomalous ridge initially forecast across the Southern Plains in
Week-2, the potential for much above-normal temperatures exists. A small area
along the Rio Grande has a slight risk of much above-normal temperatures on the
24th and 25th with the GEFS giving a 20-40% chance of daily highs exceeding 90
degrees F while also being in the 85th percentile or higher relative to
climatology. ECMWF reforecast guidance is similarly warm across the region
early in Week-2.
With anomalous troughing favored on either side of the aforementioned ridge
early in Week-2, cold is possible across both the West and Northeast. A slight
risk of much below-normal temperatures exists for the 24th and 25th across much
of the West, with the exception of lower elevations across California and the
Southwest, and for the Mid-Atlantic and Northeast. Each of these regions has a
better than 20% chance of seeing subfreezing temperatures and for daily low
temperature values to be below the 15th climatological percentile, per the GEFS
Lastly, a slight risk of heavy precipitation exists for portions of the
Middle and Lower Mississippi Valley through the the Tennessee and Ohio Valleys
on the 24th through 26th. Both the GEFS and ECMWF reforecast guidance gives a
better than 20% chance of these regions seeing an inch of rain over the three
day period and being above the 85th climatological percentile for the full
period. The enhanced odds for rainfall appear tied to ample flow of moisture
out of the western Gulf of Mexico into the highlighted region interacting with
shortwave energy across the region that may help trigger convection.
The U.S. Drought Monitor, valid on March 13, 2018 indicates an increase in
the coverage of severe to exceptional drought (D2-D4) from 15.50% last week to
16.30% this week. Severe drought continues to expand across the southern high
Plains and moderate drought areas have expanded in Georgia.
Click here to see a display of the GFS Ensemble Forecasts
Please consult local NWS Forecast Offices for short range forecasts and region-specific information.