Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 200400

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1000 PM MDT Tue Sep 19 2017

Issued at 955 PM MDT Tue Sep 19 2017

The dry cold front that race southward across the forecast area
late this afternoon has already moved well south of the forecast
area this evening. Winds will continue to weaken overnight with
relative humidities steadily rising as temperatures fall to levels
below average overnight. The higher mountains in Jackson and
Larimer Counties could see some light snow overnight with a narrow
band of baroclinicity straddling this area. Should see little to
no accumulation up there. On Wednesday, still looks like it will
not be as warm on Wednesday and winds will generally be lighter.
Although south to southwest winds will increase in speed during
the afternoon on the west slope and eastern plains. Fireweather
highlights are not anticipated for Wednesday but the wildland fire
danger will remain elevated due to low relative humidities and
dry fuels.


.SHORT TERM...(This evening through Wednesday)
Issued at 315 PM MDT Tue Sep 19 2017

A strong southwest/westerly flow aloft will be over the region
with the mid level trough axis shifting to the north and east
overnight. Weak mid level qg ascent will be around through this
evening, with moderate mid level qg descent overnight. Any
precipitation will be light through early this evening and
confined to the northern border. Strongest winds this afternoon
ahead of a cold front that will push across the cwa from the
northwest this evening. Strongest winds so far today in the
mountains, high valleys foothills and Palmer Divide. Still waiting
for stronger winds to mix out across the the northeast plains.
Reference the fire weather discussion below. The cold front is
already moving through KCYS at this time so fropa at Denver will
be much sooner, by 01z. Much faster than what the HRRR was
advertising. On Wednesday, a dry and strong west/southwesterly
flow will remain over CO with weak mid level qg descent over the
region. At 700 mb, temperatures around 12c for Denver dropping to
around 9C on Wednesday. Some relief, albeit brief, with highs near
80 in Denver.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 315 PM MDT Tue Sep 19 2017

A longwave trough will slowly shift east across western North
American through the later part of the week. Southwest flow aloft
will increase through Friday when a 110 knot jet will be over the
state. Expect very warm conditions Thursday with highs in the mid
to upper 80s across Colorado. Gusty southerly winds will increase
the fire danger over the area.

On Friday, a surface low pressure trough is expected to form over
northeast Colorado. Cooler air will be pulled in from the north.
Meanwhile southerly flow south of it will keep warm temperatures
in place. Biggest questions is where this will set up. Cold air
usually wins out and advances farther south than the models show.
Will trend the forecast this direction with cool temperatures
north and warm temperatures south. Expect isolated to scattered
showers and storms Friday afternoon and evening. Best chance for
precipitation will be over northern and eastern areas where the
best moisture will be located.

The surface trough shifts to the south and east of the area
Friday night. Cool north to northeast surface winds will prevail
behind it on Saturday. Highs will be below normal Saturday with
readings mainly in the 60s across northeast Colorado. Lift from a
short wave trough in the southwest flow aloft is expected to
increase moisture and precipitation over the area Saturday night
and Sunday. Will have the highest pops during this time frame. If
skies remain cloudy all day Sunday, highs will likely stay in the
50s over northeast Colorado.

For Monday and Tuesday, models are not in agreement on where the
upper level trough goes. The GFS keeps the trough centered over
the Great Basin while it weakens. The ECMWF is progressive and
lifts the trough to the northern plains by Tuesday. Either way the
area will be under some sort of troughiness. Therefore, will keep
temperatures on the cool side will low pops.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 955 PM MDT Tue Sep 19 2017

VFR conditions likely for the next 24 hours. Light south-
southeast winds overnight will generally remain southerly during
the day Wednesday. However, wind speeds are expected to increase
in the afternoon with possible gusts to around 25 kts on the south
and east sides of the metro area.


Issued at 315 PM MDT Tue Sep 19 2017

Will continue with the Red Flag Warning at least through this
aftn. The cold front will move into the Denver area by 01z which
should allow for temperature and rh recovery to occur a little
faster this evening. Strongest winds over the higher terrain
today. Slower to mix out across the plains but relative humidities
have dropped into the single digits. No fire weather highlights are
anticipated on Wednesday but the fire danger will remain elevated
due to the dry conditions. Less wind, cooler temperatures and
slightly better relative humidities expected Wednesday afternoon.




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