Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 180905

Area Forecast Discussion
National Weather Service Denver/Boulder CO
305 AM MDT Tue Apr 18 2017

.SHORT TERM...(Today and tonight)
Issued at 305 AM MDT Tue Apr 18 2017

Southwest flow aloft will bring mild and dry conditions to the
area today. A surface trough will form this afternoon along the
base of the foothills. This will produce southeast winds today for
most of the Front Range and eastern plains. As the trough shifts
east, winds will turn west to southwest behind it. May see some
wind gusts to 25 mph. This combined with relative humidities will
increase the fire danger. However, winds and relative humidities
are not expect to reach Red Flag Warning criteria.

A short wave trough will move into Wyoming tonight. Flow around
the base of the trough will increase with a jet max of 100 knots
moving over western Colorado overnight. This will help transport
moisture into the area. Northern Colorado and Wyoming will be
under the left exit region of the jet. This combined with lift
from the wave is expected to produce showers and thunderstorms.
Most of this should occur over Wyoming, though a few may form over
far northern Colorado. Will have low pops for this evening and
overnight for this. Over the northeast plains, there may also be
enough instability for thunderstorms.

.LONG TERM...(Wednesday through Monday)
Issued at 305 AM MDT Tue Apr 18 2017

There is a jet maximum over the CWA on Wednesday. It is due
westerly but becomes west-southwesterly Wednesday evening and
decreases a tad. Speeds are 115 to 90 knots those two periods.
West-southwesterly flow aloft continues on Thursday and Thursday
evening. All the models show an upper trough to move into western
Colorado overnight Thursday. At 12z Friday morning, the upper
trough axis is over the western half of the state. Most of the
models show a broad closed circulation with the trough either over
Colorado or to the immediate north over Wyoming at 12Z. This
feature as been moving north for the last several model runs.
Downward QG synoptic scale energy is progged on Wednesday and
Wednesday evening, then upward motion is noted early Thursday
morning well into Friday morning. There are northwesterly low
level winds that come in behind a weak cold front for Wednesday.
There are north and northeasterlies progged Wednesday night...with
east and southeasterlies Thursday and Thursday night. With all
the models continually trending north with the upper features each
successive run, you wonder where they will show the feature by
the 12Z runs on Thursday morning? That is five more major runs
from now. The only thing consistent is that all of them are
showing this northerly progression. For moisture, there is some
over the mountains early Wednesday. Moisture increases for all
areas Wednesday evening, then decreases again early Thursday
morning and for much of the day. Moisture increases again Thursday
night. Again, consistency is poor. On Wednesday, there is a
little CAPE over the mountains. The CAPE is better Thursday and
Thursday evening, but for the western 2/3rds of the CWA only. The
measurable precipitation on the QPF fields is less on this latest
run compared to previous runs. There is a bit of measurable
precipitation over the alpine areas Wednesday. There is nearly
none anywhere on any of the models Wednesday night. There is more
widespread measurable precipitation noted by late Thursday and
Thursday night. Certainly less then previous runs. The latest
Forecast Builder is still putting "likely" and even higher pops in
for Thursday night and Friday. Friday is day four and there is
80% plus pops over the plains of the BOU CWA. That is quite
optimist around here even under the best of circumstances with
impeccable consistency both run to run and model to model. But as
is always pointed out, 0.01 inch of precipitation measured is all
that is needed for the 100% pop to be "right on". For temperatures,
Wednesday`s highs are down 4-10 C from today`s expected highs.
Thursday`s highs are a only a tad lower than Wednesday`s. For the
later days, Friday through Monday, the upper trough/closed low
moves across the CWA on Friday, then strong northwesterly flow
aloft Friday night and Saturday with an upper ridge late Saturday
through Sunday. The GFS has a weak upper trough for Sunday night;
the ECMWF has zonal flow. Monday has westerly or northwesterly
flow aloft depending on the model. The GFS has more moisture, of
course, for Sunday afternoon into Monday. Friday is the coldest
day and both Friday and Saturday will see below normal readings.


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 305 AM MDT Tue Apr 18 2017

VFR conditions will prevail the next 24 hours with high clouds
increasing today. Northeast winds are expected to slowly turn
clockwise and end up southeast by 15z. Southeast winds will
continue through the afternoon with gusts up to 25 knots. A
surface low will form over the Denver area and may bring variable
winds 21Z to 24Z. Eventually the low moves off to the east and
winds will become westerly by 03Z. Winds will then settle at a
southwest drainage direction by 06Z.




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