Area forecast discussion
National Weather Service Hastings NE
1245 PM CDT Friday may 24 2013
Update...
issued at 1245 PM CDT Friday may 24 2013
Radar shows scattered showers moving out of northwest Kansas...associated with weak
shortwave trough moving through. Not sure if these are are elevated or
near surface based yet but we/re likely to find out. We will be
monitoring these showers for possible upscale growth into thunderstorms this
afternoon. Dewpoints are forecast to increase into the upper 50s-low 60s. Did
increase high temperatures with this update. Combined with highs in the
70s...MUCAPE will be 1000-2000 j/kg. An isolated severe thunderstorm is not
out of the question.
Update issued at 920 am CDT Friday may 24 2013
Forecast thunderstorm activity is not evolving as expected. So probability of precipitation were
slashed through noon. 12z lbf sounding shows a cap for elevated
parcels just below 600 mb. We are seeing showers develop over northwest Kansas.
A subtle shortwave trough is over eastern Colorado in WV imagery. Tough to get a
feel for how things evolve today. Observed and forecast soundings show
a lot of cinh in the 800-700 mb layer. Believe we will have to
watch potential for an isolated thunderstorm or two based on multiple
models attempting to fire convection.
Also decreased forecast cloud cover east of Highway 281 per visible satpix.
Wind: the pressure gradient is tight. Winds were increased through 23z.
&&
Short term...(today and tonight)
issued at 455 am CDT Friday may 24 2013
Focus is on thunderstorm potential through tonight.
The pattern aloft featured a closed upper low pressure system in the
Pacific northwest...ridging across the interior Continental U.S....and a trough
to the east through the Ohio Valley. At the surface...low pressure was
located in The Rockies...with southeasterly low level in place across our
region. Satellite imagery indicates low cloud cover increasing in
coverage along the High Plains and into western Nebraska/Kansas as moisture
advects northward.
The low cloud cover will continue to develop/expand across
central/eastern Nebraska/Kansas during the pre dawn hours with chances for
showers/thunderstorms increasing toward daybreak in our western zones in
warm air advection/Theta-E advection...with the convective active spreading
east/northeast today. An instability axis of around 1000 j/kg along
the High Plains early this morning will migrate eastward...and may
top 1000 j/kg across our western zones by evening...while shear is
around 30kts. In increasing instability and decent shear...we could
see a few strong storms.
The surface gradient tightens by middle day and south/southeasterly winds
will be on the increase with conditions at least breezy in the 15 to
25 miles per hour range...and potentially higher with gusts of 30 miles per hour or so.
Have kept temperatures right around 70f for highs today which is a
compromise between guidance with NAM/met suggesting temperatures warmer than
GFS/mav. Today is not looking like a rainout but there will be
scattered convection around.
For tonight...have followed closer to the 3km WRF...and the
GFS...Gem regional with convection developing along the High
Plains...with potential for storms to organize and roll eastward as
an mesoscale convective system overnight. Instability overnight is prognosticated around 1500 j/kg
with shear again remaining around 30kts...and again strong to severe
storms are possible with hail/wind the primary hazards.
Precipitable water values are just over an inch and some decent
rainfall amounts may be realized if storms develop as models
suggest. For lows...we are looking at milder temperatures due to the
increase in moisture/dps with readings expected to be around 60f.
Long term...(saturday through thursday)
issued at 455 am CDT Friday may 24 2013
Needless to say...this is shaping up to be a very active 6-day
period...with several...off-and-on chances for thunderstorms...and
likely at least a few rounds of severe storms and locally heavy
rainfall...as we finally get into a pattern with convective
instability consistently climbing at/above 1000 j/kg. On a
positive note...this weather pattern should result in decent
cumulative rainfall totals for all/parts of the County Warning Area through the
next week...aiding drought relief...the downside from a forecast
perspective is a lack of confidence in day-to-day details...with
convective chances for one particular day/night likely influenced
quite a bit by how the previous day/night convection or lack
thereof influences surface boundary postion...instability...etc.
As a result...despite the multiple periods of thunderstorm chances
in the current forecast...we really take this one day at a time
in terms of possible severe weather timing/placement. Would not be
surprised though to see several upcoming days eventually assigned
a Storm Prediction Center slight risk for parts of the area...and our local hazardous
weather outlook will at least attempt to give some enhanced detail
to the day 1-2 time frame.
Looking over this time frame from a broad...synoptic scale
perspective...the latest European model (ecmwf)/GFS runs are in pretty good
agreement in gradually transitioning middle-upper level flow over the
Central Plains from quasi-zonal this weekend...to more
southwesterly by middle-week as a longwave trough migrates into the
central Continental U.S. From the western states. Prior to the arrival of the
primary trough...multiple...low amplitude disturbances will track
into the plains and interact with a generally east-west low-level
front that will likely be meandering north-south across the region
due in part to convective outflow influences. As already alluded
to...the daily details are going to be difficult to discern very
far in advance. As is often the case in these patterns...deterministic
models such as the NAM/GFS sometimes develop suspect quantitative precipitation forecast bullseyes
and struggle with convective feedback issues.
Attempting to give at least a small degree of greater detail to
Saturday/Saturday night...generally prefer the 00z 4km-WRF
simulated reflectivity solution of potential Friday night
convection gradually exiting eastward out of the County Warning Area in the
morning...with a lull then ensuing for much of the day as capping
holds new development at Bay. However...by the middle and especially
late afternoon time frame...there is a least limited potential for
isolated to scattered storm development in the vicinity of an
west-northwest to east-southeast oriented warm front that sets up
somewhere across central/northern Nebraska. With the NAM
advertising 0-1km MLCAPE on the order of 3000 j/kg in the presence
of modest 0-6km bulk shear around 30kt...fairly slow-moving severe
storms and maybe even a supercell or two is possible...especially
within the northeast 1/2 of the County Warning Area...and seemingly less likely
across the southwest 1/2 including Kansas zones due to stronger
capping away from the warm front. Depending on whether or not the
warm front actually sets up across the County Warning Area...may even have to keep
an eye out for a low-end tornado threat given decent low level
shear...but this risk still seems low enough to omit from the
hazardous weather outlook. Heading into the evening and overnight
hours...its a tough call whether or not convection continues
focusing within the County Warning Area...or instead rides more so just North/East
of our area on the nose of a broad 30-40+ knots low level jet. Due
to these uncertainties...felt it would be best to at least
temporarily reduce probability of precipitation Saturday night to 50 percent across the
northeast County Warning Area...with even lower chances southwest. Although the
previous 60 probability of precipitation in the northeast were only low-end likelies...it
just seems too soon...even at this time range...to assign the
likely terminology to any given area. Turning to other elements of
Saturday weather...decent southerly breezes sustained 15-20 miles per hour
will prevail most areas...with higher speeds to around 25 and
maybe even nearing 30 miles per hour focused across Kansas zones south of the
warm front. Kept high temperatures similar...ranging from near 80 far
northeast to near 90 far southwest...although the NAM would
suggest that more of the area could end up in the upper 80s/near
90 than currently depicted.
Sunday/Sunday night...once again it GOES without saying that
convective potential will likely be somewhat influenced on how
Saturday night plays out...but otherwise its much the same story
although possibly with a bit stronger capping inferred by 700mb
temperatures climbing to around 12c in much of the County Warning Area. Like Saturday
though...the best storm chances should favor the evening/overnight
versus the daytime hours...and thus have only 20-30 probability of precipitation during
the day...versus 50s Sunday night when convection could develop
overhead along the low level jet...roll in from the west in the
form of a mesoscale convective system...or both. Again
though...most storm activity would tend to favor Nebraska versus
Kansas.
In general...this same pattern repeats itself during the Monday-
Tuesday timeframe. Then by Wednesday-Thursday...there may be a
risk for a somewhat more organized synoptically-evident severe
weather threat as the larger scale trough to the west approaches
and swings across the plains...enhancing middle-upper level flow and
resultant wind shear. However..its far too early to dive into any
more details at this time range.
As for temperature trends through the Sunday-Thursday time frame...made
little change...with highs averaging somewhere in the 80s...and
lows mainly low-middle 60s.
On one last note...although localized flooding issues certainly
cannot be ruled out at times during the next week...especially if
the same counties receive 1-2+ inches of rain two or more
days/nights in a row...we are at least heading into this pattern
with a pretty decent capacity to accept water given below normal
rainfall across the majority of the County Warning Area so far this month. As a
result...rfc 3-hour flash flood guidance is averaging at least 2-3
inches across the entire County Warning Area.
&&
Aviation...(for the 18z tafs through 18z Saturday afternoon)
issued at 1245 PM CDT Friday may 24 2013
Rest of this aftn: VFR ceilings 6-7k feet. There is a low probability for a
shower or thunderstorm. Did not include it in the taf as the exact timing
and occurrence is low confidence. South-southeast winds will continue gusting as
high as 30 kts.
Tngt: there is a risk for thunderstorms to move out of western Nebraska and reach
the terminal. This is highly uncertain though. Have indicated MVFR
stratus developing as moisture increases. If this materializes there
is potential for further decay to IFR. Current feeling is that
rain may need to occur as a prerequisite. Any clearing behind rain
that might occur and we could have a fog problem.
Sat through 18z: it all depends on what transpires tonight. If IFR/MVFR
stratus develops...expect slow improvement beginning 16-17z.
Ceiling confidence: high through 03z then low
visibility confidence: high
wind confidence: high
weather confidence: medium
&&
Gid watches/warnings/advisories...
NE...none.
Kansas...none.
&&
$$
Update...halblaub
short term...Fay
long term...pfannkuch
aviation...halblaub
