Area forecast discussion 
National Weather Service Phoenix Arizona 
940 am MST sun Mar 1 2015 


..winter weather event across affecting the Desert Southwest 
through late Monday... 


&& 


Synopsis... 
low pressure will continue a slow approach of the forecast 
area...bringing moisture into the region along with increasing 
chances of showers today and Monday. Periods of heavy rain are 
possible at times especially Sunday night through Monday afternoon. 
Cooler temperatures will also accompany this storm with below normal 
readings expected Monday through middle week. A return to above normal 
temperatures is likely by late week. 


&& 


Discussion... 
upper low centered just west of Santa Barbara this morning with the 
positively tilted trough beginning trending toward a more neutral 
orientation. Another systems is evident upstream centered near the 
Alaska/Canada border. As the trough swings around...cloudiness over 
the southeastern half of Arizona has been slowly thinning and 
retreating northward. The associated frontal zone is quite evident 
in infrared imagery extending from the Mohave desert southwestward for a 
good distance offshore. The 300 mb jet maximum was 110-120 kts per plot 
data and was located over the Mohave desert. Some subtle diffluence 
was also evident over portions of northern Arizona with shower 
activity as a result. 12z NAM and GFS initialized this divergence 
fairly well. Showers have been filling in over far southeast 
California as deeper moisture has finally pushed over the southwest 
California mountains. 12z models look to be in pretty good agreement 
on the evolution of the main features and as a result with the main 
pattern of storm total quantitative precipitation forecast. Greatest amount of quantitative precipitation forecast will be over the 
western Mogollon Rim and Yavapai County and portions of Mohave 
County...with northern portions of our forecast area on the southern 
end of that...plus a secondary maximum near and west of the lower 
Colorado River valley which was seen in previous runs. For 
south-central Arizona...including metropolitan Phoenix...anticipate only 
isolated light showers rest of today through the evening before rain 
begins in earnest early Monday and tapers off Monday evening. Might 
even need to trim probability of precipitation even more for today. Further west...the more 
significant precipitation starts today. Given the east-west timing 
differences...contemplating making adjustments to the Flood Watch to 
delay the onset for south-central Arizona by at least 6 hours. Also 
contemplating expansion of the watch to the lower Colorado River 
into southeast California. Main threat area for flooding will be 
higher terrain and nearby foothill areas. More later. 


&& 


Previous discussion issued 243 am MST/143 am PST... 
early morning water vapor imagery depicts a slowly retrograding area 
of low pressure off the Southern California coast...with quite a bit 
of middle and high cloud cover developing downwind of the coastal 
mountains along the lower Colorado River valley. Finally after much 
anticipation...precipitation appears to be developing across the western 
portions of the forecast area as radar returns have increased 
substantially since 07z. Most the the precipitation that is reaching the 
ground is still confined to the higher terrain north of Phoenix but 
nonetheless it's a good sign that the atmosphere is responding to 
the subtropical moisture that's being advected into the area. 


Low pressure will continue its slow trek down the California coast 
today...eventually stalling out west of San Diego this afternoon. In 
the meantime it will continue tapping into the anomalously moist 
subtropical plume of moisture that is evident on water vapor imagery 
south of the Baja California spur. Rain shadowing downwind of the coastal 
mountains will preclude widespread precipitation from developing this 
morning...but eventually enough moisture will build up across 
southeast California and southwest Arizona that showers will become 
more widespread in nature. The inherited forecast already depicted 
this scenario and I only made minor changes to the forecast for the 
western forecast area. To the east however...it still looks like 
precipitation will be slow to develop around the Phoenix area...possibly 
holding off until Sunday evening/early Monday morning. Latest naefs 
guidance and the past 2-3 runs of the sref indicate the best chance 
of precipitation will come after 00z tonight and thus I trimmed probability of precipitation a bit 
for today. Only thinking we will see 20-30 percent chances around 
the metropolitan...certainly looking at higher chances in the foothill 
locations north/west/east of Phoenix but it looks more and more 
likely that the more widespread rain will be delayed a bit. 


The quasi-stationary low west of San Diego will get kicked through 
the area tomorrow by a rapidly approaching secondary trough moving 
through Nevada. This will bring more widespread precipitation through the 
entire forecast area Monday morning/Monday afternoon...with rapid 
clearing from west to east Monday night. While 500mb temperature 
forecasts have risen over the past few days...bufr soundings 
indicate enough instability potential coincident with the passage of 
the trough to warrant at least a slight chance of thunder. Once the 
trough axis moves into New Mexico Monday night...precipitation chances /and 
the potential for thunder/ will drop considerably. As for storm 
total precipitation values...ensemble mean is around 0.75 inches along and 
north of I-10 with closer to 1-1.5 inches in the orographically 
favored upslope areas across northern La Paz County to 
Wickenburg...Cave Creek...and the mountains of southern Gila County. 
Made some slight adjustments to storm total quantitative precipitation forecast based on the slower 
arrival of the heavier precipitation but still enough to keep the Flood 
Watch intact. 


The impacts associated with the secondary trough on Tuesday have 
been the source of much debate due to varying model solutions...and 
individual ensemble members are still all over the map this morning. 
No doubt there will be some lingering moisture in the area...the 
question is if there will be enough forcing with this secondary 
trough to trigger showers across the area. Felt it prudent to keep 
the chance of precipitation lingering across the eastern half of the 
forecast area on Tuesday although most of the showers should be to 
the North/East of Phoenix. 00z deterministic models have trended 
drier on Tuesday...will need to see if this trend continues and if 
the ensembles pick up on this drier solution. 


From middle-week Onward...ridging will take hold across the region and 
consequently temperatures will also climb back above normal by late 
week into the weekend. 


&& 


Aviation... 
south-central Arizona including kphx...kiwa and ksdl... 


A slow approaching cold front will continue to draw low level 
moisture into the area from northern Baja California today. Any shower activity 
through this afternoon will mainly stay to the north and west of the 
Phoenix terminals...but a few light showers may affect ksdl. Ceilings 
for now should stay above 8kt feet through the afternoon hours...but 
if any showers do occur...ceilings would temporarily drop to around 5-6k 
feet. The cold front and area of rain will approach the Phoenix area 
sometime late this evening into tonight...but confidence in the 
exact time of arrival of the rain...lower ceilings...and any restricted 
visibilities is still somewhat low. 


Southeast California and southwest Arizona including kipl and kblh... 


Middle level ceilings will gradually lower this morning and eventually drop 
to MVFR by around noon after some steady rain moves in. Confidence 
is fairly high that a band of light to moderate rain will affect 
southeast California and portions of southwest Arizona this 
afternoon into this evening. May see some MVFR visibilities at times 
due to the rain and potentially IFR ceilings briefly depending on how 
steady the rain ends up being. Rain should diminish after 09z 
tonight with gradually improving ceilings by 12z Monday morning. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
Tuesday through Saturday... 
cool and still relatively moist conditions will be present on 
Tuesday with lingering rain chances across south-central Arizona. 
Weak high pressure and drying conditions will move in starting 
Wednesday. Temperatures will warm back toward normals by Thursday 
and then a few degrees above normal for Friday and Saturday. Winds 
will be relatively light through the period...but afternoon 
breeziness is expected on Wednesday through Friday. Minimum 
humidities will drop into the 20s on Wednesday and then into the 
teens each afternoon starting Thursday. 


&& 


Psr watches/warnings/advisories... 


Arizona...Flood Watch from 5 PM today through 5 PM Monday azz021>024. 


California...none. 


&& 


$$ 


Visit US on facebook...twitter...and at weather.Gov/Phoenix 


Discussion...aj 
previous discussion...leins 
aviation...Kuhlman 
fire weather...Kuhlman 



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