Area forecast discussion
National Weather Service Hastings NE
627 am CDT Wednesday may 22 2013
Update...
issued at 625 am CDT Wednesday may 22 2013
As discussed earlier...ended up making a few adjustments to very
short term grids based on latest radar trends...and with support
from the latest rap13/hrrr qpf/reflectivity. Although cannot rule
out a few spots reporting light measurable rain with this
activity...especially in the Nance/Polk County area...think that
non-measurable sprinkles will be the dominant story for most of
the affected area...and thus left out a measurable pop.
However...beefed up sprinkle wording to widespread in several far
northeast counties through noon 17z...and expanded an isolated
sprinkle mention as far south as a Loup City-Geneva line. It may
turn out that this light precipitation may be out of the picture well
before noon...but given the very slow moving nature of the parent
middle level low to the east anything is possible. Also beefed up sky
cover percentages from mostly cloudy to outright cloudy across
roughly the northeast 1/2 of the County Warning Area through mid-day...while
leaving a more optimistic trend in place farther southwest. The
latest hrrr is now sporting a roughly 20 degree high-temperature gradient
across the County Warning Area this afternoon from only low-middle 50s northeast to
middle 70s southwest. Other short term models such as the rap13 are
not as cool in the northeast as the hrrr...and thus will resist
the urge to modify afternoon highs at this time...but for sure its
possible that some northeast areas could struggle to reach the
middle-upper 50s and next shift will have monitor trends.
&&
Short term...(today and tonight)
issued at 509 am CDT Wednesday may 22 2013
While there is certainly no high-impact weather during the next
24 hours...there are at least a few tricky elements at
hand...including the extent of low stratus and its possible
influence on a fairly decent southwest-northeast temperature
gradient...and also the extent of sprinkles/light rain showers
mainly this morning in the northeast.
08z objective surface analysis reveals no major features across
the Central Plains...with the County Warning Area positioned between a modest 1004
millibar low over eastern Iowa...and a 1014mb high over the Nebraska
Panhandle...a southward extension of a stronger...broader high
well to the north in Canada. Within the modest pressure gradient
between these features...a fairly steady north-northwest breeze of
10-15 miles per hour has prevailed through the nigh in most areas...with
occasional gusts over 20 miles per hour. Temperatures will start out the
morning seasonably cool...with eventual lows aimed into the middle-
upper 40s all areas. Aloft...water vapor imagery and short term
model data features a closed 500mb low drifting slowly
east...centered near the Iowa/SD/neb border...and a fairly sharp middle
level trough axis then trailing south-southeast into the middle-south
states. Off to the west of the local area...a low amplitude middle
level ridge is aligned north-south through The Rockies...with a
fairly vigorous...expansive closed low spinning over the Pacific
northwest. While it has taken its time forming...11-3.9 micron
satellite imagery and automated observation have finally revealed the
formation of fairly widespread low stratus mainly in the 1500-2500
feet range over much of the County Warning Area...although some breaks in the clouds
do still exist. Also just within the past 90 minutes or so...an
increasing flare-up of light rain/sprinkles has taken place mainly
over the far northern/northeast County Warning Area...with upstream observation at places
such as Oneill and Albion confirming at least trace amounts are
falling.
Today...the main story aloft involves the continued slow departure
of the closed middle level low...with the center of this feature
prognosticated to only reach northeast Iowa by 00z. At the surface...a
modest pressure gradient will remain in place as high pressure to
the north noses southward...and this in combination with mixing to
at least 850mb in most areas will result in somewhat breezy
conditions especially within the northeast 2/3 of the County Warning Area...with
sustained northwest winds 15 to possibly 20 miles per hour...and some higher
gusts. The real challenge today lies initially with the extent and
intensity of rainfall...and also just how stubborn the low stratus
deck will be...and whether or not it retreats slow northeast
during the afternoon. Starting out early this morning through middle-
day...have introduced a mention of sprinkles to a previously
precipitation-free forecast for far northeast areas mainly north of a
Greeley-Polk line...as decent low-middle level saturation up to
around 600mb resides mainly in this zone and points
northeast...associated with a batch of middle level moisture focused
in the deformation zone on the back-side of the departing low. The
very short term question that will need pondered over the next few
hours is whether to expand this sprinkle mention southwestward
closer to I-80 per suggestion of the latest hrrr reflectity
forecast...and also whether to up the ante to a measurable pop. Will
give it some time to see if area observations support a few
hundredths of accumulations before possibly making this move. At
any rate...any light rainfall in the northeast...especially of the
potentially measurable variety...is expected to be out of the
picture by noon...as the middle levels start to dry out from west to
east. Cloud-wise...have leaned toward a combination of the NAM and
hrrr...with the expectation of a rather sharp southwest-northeast
gradient ranging from mostly sunny-partly cloudy southwest to
mostly cloudy at best in the northeast. Shaky confidence in these
cloud details lead to somewhat shaky confidence in high temperatures. At
least for now...nudged southwest areas up a few degrees and
northeast areas down a few degrees versus previous...while leaving
the central rather unchanged. As a result...am aiming from upper
50s-low 60s far north/northeast to middle 60s Tri-Cities to middle 70s
southwest. However...am starting to wonder if the hrrr might be
onto something...keeping much of the northeast 1/2 of the County Warning Area well
down in the middle-50s at best under the longest-lasting stratus.
This will bear watching for later updates...but Don/T want to go
quite this cool yet.
Turning to the tonight period...00z-12z...confidence remains
rather high in a legitimately dry night...with the sole possible
exception of some patchy light drizzle in the evening in the north
under a low stratus deck...but this scenario is too uncertain to
insert at this time. In the middle-upper levels...the departing low
finally slips far enough east to allow the ridge to the west to
start building into neb/KS. Although there will be a modest
northwest-southeast oriented Wing of middle level moisture
advection/isentropic lift that sets up from southwest Nebraska to
southeast Kansas mainly after midnight as evident on the 310k
surface...soundings suggest that lift/moisture should be
insufficient for precipitation over the County Warning Area...although a few showers or
even weak storms could flare up within a few counties just to the
south/southwest by sunrise. At the surface...high pressure will
continue oozing down from the north...keeping fairly light but
steady northerly breezes around 10 miles per hour in place. Despite the
presence of potentially decent low level saturation...these
breezes should keep anything more than haze/light fog at Bay. The
real question mark at this point tonight again lies with low
stratus trends. While the previous forecast had no worse than
partly cloudy skies...the latest NAM runs and associated met
guidance fairly aggressively bring a batch of low stratus back
southward across most of the County Warning Area as the night wears on...with this
especially evident at 900mb. For now...have started an upward
trend in sky cover...but very well may not be high enough
especially Post-midnight. Even if stratus does end up being more
widespread than currently suggested...confidence is decent that
low temperatures will end up similar to those of this morning...and used
a blend of met/mav guidance...aiming for another cool one with middle
40s most spots.
Long term...(thursday through tuesday)
issued at 509 am CDT Wednesday may 22 2013
Models are in good agreement transitioning a ridge of high pressure
across the plains on Thursday in wake of the departing upper level
low. While heights aloft will be rising...expect only a modestly
warmer afternoon on Thursday as 850mb temperatures are expected to only
rebound a couple of degrees and plenty of cloud cover is expected in
easterly upslope flow.
As the winds shift more southeasterly Thursday night and
Friday...expect additional moisture to surge northward...and for
increasing instability across the region. This building instability
combined with some forcing from the low level jet...will likely
result in a chance of thunderstorms Thursday night across the
southern half of the forecast area...which will be in closer
proximity to the jet. With the nose of the nocturnal jet more
focused across The Heart of local area Friday and Friday
night...cannot rule out additional elevated shower and thunderstorm
activity across the entire forecast area...with the best chances for
precipitation coming during the late evening and overnight hours.
Thereafter the middle level cap is expected to weaken substantially by
Saturday afternoon... and with several thousand joules of cape to
work with through the remainder of the long term periods...will
likely see thunderstorms around each day as periodic vorticity
maxima eject out of the longwave trough across the West Coast.
Additionally...with these substantial cape values and marginally
supportive shear profiles...will have the potential for some severe
thunderstorms develop from time to time Saturday Onward...with large
hail and strong thunderstorm wind gusts being the primary threats.
Temperatures through the period are not expected to fluctuate a
whole lot as there will be little change in the underlying airmass
Saturday Onward...with lower to middle 80s expected each afternoon
and lower 60s expected at night.
&&
Aviation...(for the 12z kgri taf through 12z Thursday morning)
issued at 625 am CDT Wednesday may 22 2013
Although confidence is high in VFR visibility through the
period...ceiling trends continue to be tricky as plenty of lower
stratus hangs around the area on the back side of a departing middle
level low. Starting off through the morning into the middle-afternoon
hours...have continued a prevailing MVFR ceiling...although just
within the past hour clouds at the MVFR level have scattered a
bit...diminishing confidence somewhat and suggesting that MVFR
may be more off-and-on. Also this morning...a few sprinkles cannot
be ruled out at kgri...but given such low impact will omit any
precipitation mention. Through the late afternoon and evening
hours...kept trend from previous forecast by raising prevailing
ceiling to low-end VFR. However...confidence in the details really
decreases by the last 6 hours or so late tonight...as some
models/guidance suggest a low-MVFR and possibly even IFR deck
could materialize...while other models keep ceiling VFR. As a
compromise between the wide possibilities at this time
range...will introduce a scattered low-end MVFR cloud group.
Wind-wise...a steady north-northwest breeze will prevail much of
the day with gust potential to around 22kt...before slackening this
evening and averaging under 10kt overnight.
&&
Gid watches/warnings/advisories...
NE...none.
Kansas...none.
&&
$$
Update...pfannkuch
short term...pfannkuch
long term...sar
aviation...pfannkuch
