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Area Forecast Discussion (AFD)

Issued by Kansas City/Pleasant Hill, MO (EAX)

                            
000
FXUS63 KEAX 142323
AFDEAX

AREA FORECAST DISCUSSION
National Weather Service Kansas City/Pleasant Hill MO
523 PM CST Thu Feb 14 2019

.Discussion...
Issued at 300 PM CST THU FEB 14 2019

Key Points:
* Multiple rounds of winter precipitation are expected across the 
  forecast area during the next week -- tomorrow/Friday, Saturday 
  evening/night into early Sunday morning, and then around the 
  middle of next week.
* A Winter Weather Advisory is in effect for all of the NWS Kansas 
  City/Pleasant Hill MO CWA for Friday into Friday evening except 
  for Putnam, Schuyler, and Adair Counties.
* Forecast maximum snow accumulations for Friday have increased by a 
  couple of inches -- storm-total snow accumulations now expected to 
  be 2-5 inches, locally to 6 inches.
* Locations in and around the Kansas City Metro Area from west-
  central MO toward adjacent extreme eastern Kansas will be
  monitored for a possible upgrade to Winter Storm Warning -- as
  localized snowfall accumulations could reach near 6 inches,
  accompanied by 1-inch-per-hour snowfall rates from around 9-10 
  AM to 2 PM. These snowfall rates, amid the influx of a polar air
  mass, will support efficient/fast accumulations of snow on area
  roadways.
* Additional winter-weather headlines may become necessary for the 
  aforementioned subsequent winter-weather events in closer temporal 
  proximity to these events, as forecast predictability increases.

During the past 12-18 hours, moisture-channel imagery suggests that 
the exit region of a cyclonically-curved midlevel speed maximum 
crossing the far east Pacific has emerged over the southwest states. 
As a result, initial conditions for numerical weather prediction 
model guidance are now better accounting for the actual 
characteristics of this perturbation as its augmentation to 
tropospheric flow and thermodynamic fields become better resolved by 
the CONUS upper-air network. This has prompted a modest, though 
meaningful, change to the character of the perturbation as it 
advances eastward toward the central Plains. Specifically, the 
latest multi-model consensus suggests better definition to the 
wave as it quickly tracks from the southern Sierra to the Mid-
South region from this evening to Friday evening. This disturbance
will become embedded within a broad zone of falling midlevel 
heights -- in advance of a much deeper cyclonic perturbation 
amplifying over the West Coast -- suggesting its local 
augmentation to upward-motion fields amid a broader regime of 
isentropic ascent.

In response to these developments, a relatively fast-moving zone -- 
though zonally-elongated -- of forcing for ascent will transit 
eastward across the area from Friday morning into the evening -- 
atop a polar air mass that continues to surge southward -- 
characterized by surface temperatures holding in the teens across 
central Nebraska. The superposition of the upper-jet-streak- 
enhanced vorticity center and background poleward-sloping 
baroclinic zone will foster a corridor of low/mid-level 
frontogenesis from eastern KS to western MO -- which the latest 
model solutions have suggested as being more robust than 
previously anticipated. Moreover, more substantial forcing for 
ascent within the jet streak left-exit region is expected to 
phase with adequate moisture across the region to increase 
snowfall totals. Vertical cross sections across the area indicate 
a subtle zone of reduced static stability in the middle and upper 
levels -- which could amplify upward motion, thus boosting 
precipitation totals.

With midlevel steering winds being oriented largely parallel to 
the amalgamation of the elongated frontogenetic band and 
isentropic-ascent corridor, the prolonged duration of 
precipitation should offset the effect of limited moisture return
in support of substantive QPF. Moreover, given appreciably cold 
low-level air surging southward, snow-to-liquid ratios of 10:1 to 
15:1 are anticipated, resulting in a couple more inches of snow 
now expected compared to the previous forecast. This is expected 
to bring snowfall totals to 3-6 inches southwest of a line from 
Worth County to Howard County -- highest in and around the Kansas 
City Metro Area. Snowfall totals of 1-3 inches are expected 
farther northeast. With Mercer and Sullivan Counties expected to 
receive around 2 inches of snow, the Winter Weather Advisory has 
been expanded to include these counties. As a result, the Winter 
Weather Advisory now includes the entire CWA except for Putnam, 
Adair, and Schuyler Counties, and the start time of the Winter 
Weather Advisory has been changed from 9 AM to 7 AM. Forecast 
soundings suggest that mixing-ratio gains within the initially dry
boundary layer -- accompanying the antecedent polar-air influx --
will contribute to light snow reaching the surface a couple hours
earlier -- first impacting far western MO and extreme eastern KS.
Present indications are that the most-likely overlap of highest 
snow-to-liquid ratios (coldest air accompanying the surface high- 
pressure area) and strongest ascent will phase around the Kansas 
City Metro Area including west-central MO and adjacent extreme 
eastern KS. This could support snowfall accumulations approaching 
Winter Storm Warning criteria. However, given the progressive 
nature of the open wave -- despite its accompanying better 
definition and stronger ascent -- limiting precipitation duration,
and given the only-modest phasing between dendritic growth zones 
somewhat overlying stronger forcing for ascent, confidence in 6+ 
inch snowfall totals is only modest at this time. Nevertheless, a 
duration of 1-inch-per-hour snowfall rates will be possible in and
around the Kansas City Metro Area, as addressed in the key points
section, and this will have the potential to produce hazardous 
travel conditions as snow potentially rapidly accumulates over the
cold surfaces. An upgrade to Winter Storm Warning will be 
possible in subsequent forecasts across parts of the Winter 
Weather Advisory area.

Snow is expected to have exited the area during the overnight 
hours of Saturday night, giving way to drying conditions amid a 
strong surface ridge. Overnight temperatures are expected to fall 
into the single-digit readings across much of the northern two-
thirds of the CWA amid the surface ridge and a fresh snowpack -- 
encouraging ample radiational cooling. For Saturday evening/night 
into Sunday, forcing for ascent accompanying the left-exit region 
of an upper-level jet streak evolving from the aforementioned 
amplifying West Coast trough will cross the region. This will 
combine with modest isentropic ascent to support another chance 
for precipitation. Light to moderate snow will be possible, though
the modest, and at-times shallow, ascent may limit ice nucleation
in clouds -- resulting in the potential for freezing rain. Sleet 
will also be possible. Overall accumulations of precipitation 
should be minor given the lack of even stronger ascent/moisture 
return, though travel impacts will be possible.

From mid-day Sunday through early next week, slight moderation of
temperatures is expected as a long fetch of midlevel 
southwesterly flow persists over the area downstream of a 
narrowing-wavelength -- though high-amplitude -- midlevel trough. 
Initially weak forcing for ascent embedded within this pattern 
will gradually increase in magnitude, bringing a chance for winter
precipitation by the middle of next week. The overall 
predictability of the large-scale pattern appears to be higher
than normal for this forecast time range -- based on the envelope
of model solutions -- so at least modest confidence exists in 
winter-precipitation chances for the middle of next week.

&&

.Aviation...(For the 00Z TAFS through 00Z Friday Evening)
Issued at 518 PM CST THU FEB 14 2019

Stiff winds behind the recently passed frontal boundary will
gradually weaken through the evening and overnight. Expect the
current stratus deck to remain in place for another couple hours,
with the next couple hours after that showing some gradual
improvement for the overnight hours. Mid level clouds will also 
gradually build in early Friday morning, eventually bringing CIGS 
down to MVFR levels by the mid to late morning hours. Could see 
some light snow as early as 15 UTC, but the heavier snow with 
significantly reduced CIG and VIS should hold off until a couple 
hours later, more toward 17 UTC. By 22 UTC conditions should start
improving, especially the VIS, while CIGS could take a while 
longer to improve to better than MVFR. 


&&

.EAX WATCHES/WARNINGS/ADVISORIES...
KS...Winter Weather Advisory from 7 AM Friday to midnight CST Friday 
     night for KSZ025-057-060-102>105.

MO...Winter Weather Advisory from 7 AM Friday to midnight CST Friday 
     night for MOZ001>006-011>016-020>025-028>033-037>040-
     043>046-053-054.

&&

$$

Discussion...Cohen (NWS Topeka)
Aviation...Leighton