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Area Forecast Discussion (AFD)

Issued by Springfield, MO (SGF)

                            
000
FXUS63 KSGF 191113
AFDSGF

Area Forecast Discussion
National Weather Service Springfield MO
513 AM CST Tue Feb 19 2019

...Wintry Weather to Impact Afternoon/Evening Commute...

.SHORT TERM...(Today and tonight)
Issued at 201 AM CST Tue Feb 19 2019

Yet another wintry system is on the way for later this afternoon 
and tonight. A deep trough over the Rockies will slowly swing out 
into the Plains from later today into tonight. Short wave energy 
will round the base of that trough and effectively result in the 
main trough taking on more of a negative tilt.

As this occurs, an upper level jet streak will strengthen from the
Corn Belt and western Great Lakes through the northeastern United
States. The combination of the approaching trough and the right-
entrance region of this jet streak will keep mid and upper level
lift plentiful across the Ozarks today. While we could see a few
flurries from this morning into early this afternoon, the main
impact of this higher level lift will be mostly cloudy skies.

The main show will come starting later this afternoon as
very deep isentropic upglide develops in the lower and middle
levels of the atmosphere. With the upper level lift already in
place, we expect precipitation to rapidly develop and spread north
across the Missouri Ozarks and southeastern Kansas. While there
will be some dry air to overcome in the lower levels of the
atmosphere, this dry air will be no match whatsoever for this deep
tropospheric ascent.

Close inspection of temperatures and moisture profiles using both
the top down and Bourgouin (energy) methods indicates that 
precipitation will start as snow over most areas. There could 
initially be some rain/sleet mixed in at onset, but wet-bulbing 
should change everything to snow in rapid fashion. 

Speaking of wet-bulbing, high temperatures over most areas will 
occur in the late morning and early afternoon. Temperatures will
then fall as precipitation starts. We have gone with the coldest
end of temperature guidance (HREF) given expected cloud cover and
a persistent east wind.

What is particularly concerning is both the onset time of snow
later this afternoon and the expected snowfall rates. Despite 
snow to liquid ratios of less than 10 to 1 and the dendritic 
growth zone layer being rather high in the atmosphere, the depth 
and magnitude of upward omega should result in snowfall rates over
0.5" per hour with some locations approaching 1" per hour late
this afternoon and early this evening. This will likely result in
the potential for significant impacts to the evening commute. 

As we head into this evening, a warm nose will rapidly advect
north into much of the Ozarks as a strong low level jet develops.
We went with the more aggressive NAM model regarding the 
northward extent and strength of the warm nose. We are therefore 
expecting snow to change to sleet and then freezing rain from
south to north throughout the evening. There are a few additional
items to complicate matters even more this evening:

1) The surface freezing line will also slowly move north across
southern Missouri. This will result in freezing rain changing to 
rain.

2) Elevated instability will be present across south-central 
Missouri. This may have an impact on precipitation type. For 
example, pockets of sleet may develop within the larger area of 
rain/freezing rain.

As we then get into later tonight, confidence is high that a dry
slot will work northeast through the area and result in a change
over to freezing drizzle and drizzle. That freezing line will
continue to slowly move north through the night, thus the area of
potential ice accretion will tend to shrink with time.

As for total snow accumulations, we are expecting 1" to 2.5" of 
snow over much of the area. Locations right along the Arkansas 
border may see slightly less due to a quicker changeover.

Ice amounts are expected to range from a thin glaze to as much as
two-tenths of an inch. We will have to watch areas around West
Plains closely as forecast amounts in that area are not too far
from warning criteria.

The plan at this point is to leave the current Winter Weather
Advisory intact. We tinkered with the idea of starting it a bit
later over central Missouri, but decided not to get too cute with
a few models (like the Canadian) starting precipitation by 21Z.

.LONG TERM...(Wednesday through Monday)
Issued at 201 AM CST Tue Feb 19 2019

The initial concern will be whether or not the threat for
drizzle/freezing drizzle is over by 12Z on Wednesday. We have
maintained a mention of patchy drizzle/freezing drizzle through 
mid-morning, especially across central Missouri. If confidence in
this scenario increases, the advisory may need to be extended for
a few hours. We have again gone on the cold side for temperatures
on Wednesday with expected snow and ice on the ground over many 
areas.

Upper level ridging will then bring a stretch of quiet weather 
from later Wednesday through Thursday night. Temperatures will be
fairly close to normal with highs ranging from the middle 40s to 
lower 50s.

Global models then indicate a deep trough ejecting out of the
western United States from later Friday into early Saturday.
Models are fairly consistent with short wave energy rounding the
base of the trough and taking on a negative tilt as it passes
through the central Plains. At the surface, strong low pressure 
is forecast to track from the Texas Panhandle region into the 
Kansas City area before moving into the southern Great Lakes (a 
classic panhandle hook).

This track would largely put the Ozarks region into the warm
sector of this system with increasing chances for showers and
thunderstorms. The coverage of thunderstorms (as well as the
potential for severe storms) will largely be a function of how
much Gulf of Mexico moisture can advect this far north.

We should see above normal temperatures late this week in the warm
sector of that system. Temperatures will then cool down slightly
later this weekend behind the departing system, however we are 
not looking at any sort of Arctic intrusion.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 507 AM CST Tue Feb 19 2019

VFR conditions are expected through early this afternoon with
increasing easterly surface winds.

Ceilings will then lower late this afternoon with snow developing.
IFR will become highly likely by sunset due to falling snow and 
lowering ceilings.

Precipitation will then change to sleet and freezing rain
throughout the evening before then changing to freezing drizzle
and drizzle overnight. IFR and areas of LIFR conditions are 
expected overnight.

Low level wind shear conditions are also expected from late this
afternoon into tonight.

&&

.SGF WATCHES/WARNINGS/ADVISORIES...
MO...Winter Weather Advisory from noon today to 6 AM CST Wednesday 
     for MOZ055>058-066>071-077>083-088>098-101>106.

KS...Winter Weather Advisory from noon today to 6 AM CST Wednesday 
     for KSZ073-097-101.

&&

$$

SHORT TERM...Schaumann
LONG TERM...Schaumann
AVIATION...Schaumann