Scientific Forecaster Discussion

Return to Local Conditions & Forecast
Area forecast discussion 
National Weather Service Hastings NE
904 PM CDT Wednesday Apr 23 2014

issued at 904 PM CDT Wednesday Apr 23 2014

As anticipated...most storms in the County Warning Area have weakened quite a bit
in the last 90-120 diurnal cooling and outflow-
dominant Mode has resulted in decreasing overall
instability...with latest Storm Prediction Center hourly mesoscale-analysis suggesting
mixed layer cape is down into the 500-1000 j/kg range across the
southern County Warning Area...and considerably lower within much of the northwest
half. The eastward-moving outflow mainly within 30 miles either
side of the state line is being closely monitored for gusts in the
50-60 miles per hour range per an earlier observation at Phillipsburg Kansas
Airport...but suspect most peak gusts are now holding at least
slightly into sub-severe range mainly in the 40-55 miles per hour range. At
least for now...the Severe Thunderstorm Watch will be allowed to
ride...but the obvious reality is that for most of the County Warning Area the
primary threat has passed.

As for rainfall amounts...we have good reason to believe based on
a few ground-truth/automated measurements that radar-derived
estimation so far during this event has largely run at least
twice...if not locally 3 times higher than reality even per the
dual pol products within the heaviest rain bands...mainly due to
significant hail contamination. In other words...although many
areas within the main corridor of storms have likely received 1-2"
from this event and maybe a few isolated spots closer to 3"
not believe anybody has received 4+" as some of the radar products
suggest. Given the antecedent dry conditions within the area of
highest rainfall...we strongly suspect that Hydro issues have been
held to a minimum...and thus have refrained from any Flood
Advisory/warning products thus far. Of course...we will have to
wait until Thursday morning to receive true ground truth rainfall
amounts from dozens of National Weather Service cooperative
observer...cocorahs...nerain et cetera. Observers.

Update issued at 725 PM CDT Wednesday Apr 23 2014

Just updated forecast to attempt to fine-tune short term
convective trends a little bit. As is often the case during the
first few hours of an event along a focused frontal zone...the
vast majority of storms have concentrated within a
narrow...generally no more than 10-20 mile wide southwest-
northeast corridor cutting across The Heart of the County Warning Area from
generally the Beaver City area in the southwest...through the
Minden area through the Doniphan-Hastings area through the Polk
County area. Within this zone...there have been numerous reports
of hail to at least penny size...and isolated to around Golf Ball
size or slightly larger. Although likely overdone at least
modestly by hail contamination...some areas especially near the
Franklin/Phelps/Harlan/Kearney County border area have likely
picked up 2-3 inches of rain...while outside the main corridor the
majority of the County Warning Area has struggled to see a drop of rain thus far.

Based on the latest trends in higher res models such as the
hrrr/rap/4km WRF-nmm...a gradual weakening trend in storms should
occur over the next few hours...but as this occurs there should
also be a gradual expansion in areal coverage of convection as the
main cold front pushes in from the northwest. Severe Thunderstorm
Watch continues as originally issued for entire County Warning Area at this time.


Short term...(this evening through thursday)
issued at 427 PM CDT Wednesday Apr 23 2014

Focus is on thunderstorm development and the potential for severe
thunderstorms late this afternoon/evening. So far events are
unfolding as expected with early day convection in our eastern
zones...with redevelopment of storms expected during the next few
hours across our western County Warning Area along ahead of a cold front and dry
line. Aloft...the upper trough extended from British Columbia
southeastward across the Rocky Mountains with southwest flow in
place across the Central Plains. At the surface...low pressure
centers were located in western ND...NC Nebraska and northwest Kansas. Moisture
has been on the increase today with dps currently in the middle 40s to
around 50f and latest Storm Prediction Center mesoscale Page places an instability axis of
around 1000 j/kg just ahead of the dry line across western Kansas.
Model forecasts an axis of instability of around 1500 j/kg extending
north through NC Kansas into SC Nebraska this evening...and deep layer shear
on the order of 30 to 40kts. It is just a matter of time before we
see storms fire along the boundaries. Individual storm motion will
be to the northeast while the entire convective area shifts eastward
with time as the boundaries advance this evening/tonight.

Severe storms remain possible from late afternoon through the
evening...before instability decreases overnight. Large hail the
size of quarters and damaging winds of 60 miles per hour still will be the
primary hazards from the storms. Widespread rainfall looks
promising with rain amounts around one inch or so not out of the
question. Not expecting Hydro concerns at this time given how dry things
have been.

Convection is expected to be widespread during the evening and
overnight hours along the cold front and as the middle/upper wave move
through. The cold front is prognosticated to be southeast of our County Warning Area by
12z Thursday with the middle level trough axis just behind. There may
be some lingering showers for an hour or two Thursday morning before
things rapidly dry out behind the system.

Also tonight...there are good pressure rises behind the cold front
and we could see a period of gusty northwest winds and will monitor to see
if current Wind Advisory needs to be expanded or extended for the
Post frontal northwest winds.

Long term...(thursday night through wednesday)
issued at 427 PM CDT Wednesday Apr 23 2014

To summarize in the shortest way possible...this 6-day period is
highlighted by a potentially rather active precipitation pattern
at various points between Saturday and Tuesday...and possibly even
some increasing potential for at least 1 or 2 rounds of severe
thunderstorms mainly during the Saturday afternoon through Sunday
evening time frame. Although still subject to considerable
uncertainty...the synoptic pattern favors Sunday afternoon/evening
for the potentially highest severe threat. Although these details
will obviously be hammered out in coming days...this weekend
severe potential which has already highlighted by Storm Prediction Center in the day
4-8 outlook...has now been added to our local hazardous weather
outlook (hwo) product as well. Temperature-wise...a gradual fall
is advertised from highs in the 70s/near 80 during the Friday-
Saturday time frame to only the 50s/near-60 from Mon-Wed.

As for non-thunderstorm hazards or potential hazards that could
also end up in the severe weather potential statement eventually but are not there yet...and
going in chronological order: 1) will have to keep an eye on
Friday afternoon for at least near-critical fire danger. See
separate fire weather section below for further detail on this. 2)
going much farther out in the forecast...there are already decent
signals that strong winds potentially near-to- meeting advisory
criteria of sustained 30+ miles per hour will develop out of a northerly
direction during the Monday-Tuesday time frame in the wake of a
strong low pressure system. Although the official forecast already
has sustained speeds worthy of an severe weather potential statement mention...given this is
still 5-6 days away will omit from now and wait until it gets a
bit closer in time/confidence grows.

Getting into some abbreviated detail given ongoing active
weather...and taking it mainly in 36-48 hour blocks...

Thursday evening through Friday night...this 36 hours remains
rather high confidence in dry a period of
shortwave ridging dominates the Central Plains in the middle-high
levels in between low pressure systems. As already
mentioned...confidence even in wind direction for Friday is a bit
lacking...but temperature-wise...bumped up both Thursday night and
Friday night lows and Friday highs generally 2-3 degrees from
previous forecast based on blend of latest guidance/models.

Saturday daytime through Sunday night...there is increasing
confidence that at least 1 round of severe thunderstorms could
occur during this time frame...but we are a few days away from
gaining much confidence in the details. From a large-scale middle-
upper level perspective...both the 12z European model (ecmwf)/GFS are in pretty
good agreement for this time range taking a strong...amplified
trough from a position centered just off the cal coast on
Saturday morning...then swinging it into the central/southern
rockies in a negatively tilted fashion by Sunday morning...and
then closing it off over Kansas or vicinity through Sunday night. As
can be expected with this type of system...a potentially rather
strong surface low should wind up over the High Plains generally
along the Kansas/co/neb border area...drawing increasing moisture into
the area and possibly setting up a fairly sharp warm front. Only
have slight 20 probability of precipitation advertised for many models show
forcing insufficient for thunderstorm initiation...but should
something fire severe is possible. Probability of precipitation increase into the 30-50
range for Sat night through Sunday night as large scale forcing
increases considerably. Actually...the biggest change of all in
the entire 7-day forecast may be for high temperatures both Sat and they are now trending quite a bit warmer. Although did
not get carried away given its still 3-4 days out...raised highs
both of these days generally 3-6 degrees from previous
forecast...with the latest NAM even suggesting widespread 80s for
Saturday. Stay tuned for additional possible upward adjustments.

Monday daytime through Tuesday daytime...left a smattering of
20-40 probability of precipitation going as the now very deep low slowly moves east of the
region. Have no thunderstorm mention beyond precipitation
should trend more toward generic rain showers...although as
previously mentioned strong winds could become an issue. Temperatures not
changed very much either day...with highs mainly 50s.

Tuesday night/Wednesday daytime...left it dry for now as large
scale forcing heads farther east...but not necessarily a guarantee
to stay this way. High temperatures Wednesday very preliminarily aimed into
middle-upper 50s.

Lastly...although have no low temperatures in the official 7-day forecast
that could support frost formation...we are currently staying just
barely above frost thresholds on Monday and Tuesday nights in some this may bare watching depending on what wind speeds


Aviation...(for the 00z kgri taf through 00z Thursday evening)
issued at 725 PM CDT Wednesday Apr 23 2014

The vast majority of aviation issues during the next 24 hours will
occur during the first 5-9 hours as fairly widespread
shower/thunderstorm activity tracks near...and at times directly
over kgri...potentially resulting in brief MVFR visibility
restrictions in the heaviest downpours...and possibly small hail.
Have run a tempo group for the initial 4 hours to capture the most
likely time frame for widespread activity...with only a vicinity
thunderstorm mention thereafter...although future taf
issuances/amendments may very well extend tempo/prevailing groups
farther out in time. Have reasonable confidence that all
precipitation should be out of the area by around 10-11z on
Thursday morning...with high confidence in dry conditions
thereafter through the Thursday daytime hours. Wind-wise...kgri
will be prone to potentially erratic/variable winds with gust
potential to over 30kt during these first few hours in
convection...but by later this evening/tonight a prevailing
northwesterly wind will take over behind a cold front...with
sustained speeds through much of the period averaging at least
15-20kt with frequent gusts 24-30kt.


Fire weather...
issued at 350 PM CDT Wednesday Apr 23 2014

On Thursday...much drier air advects onto the
Central Plains behind the cold frontal boundary with dewpoints
falling into the 20s during the afternoon. Northwest winds will
be gusty during the morning however wind speeds will gradually
decrease during the afternoon as the surface gradient relaxes. If
we receive widespread...and good rainfall amounts...fuel
conditions will remain unfavorable in Kansas. Officially fuels
remain favorable in Nebraska even if we receive rainfall tonight.
Current relative humidity values are forecast to drop near or
below 20 percent across areas mainly west of Highway 281 with near
critical relative humidity farther east. Given uncertainty in strength of winds
in the latter afternoon hours...will hold off on Fire Weather
Watch at this time...but this will need to be monitored closely.

For Friday afternoon...although confidence has risen that
relative humidity values will drop down into the 20-25 percent
near-critical range across nearly the entire County Warning Area...the
Colorado-location of sustained winds/gusts of at least 15/20 miles per hour are
very much in even two days away models such as the
NAM/GFS are unusually out of sync on wind speed and even
direction. Not to mention...those areas that see appreciable rain
during the next 12-15 hours could have fire danger mitigated
these next few days.


Gid watches/warnings/advisories...


short term...Fay
long term...pfannkuch
fire weather...Fay/pfannkuch