000 
fxus66 ksgx 170422 
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Area forecast discussion 
National Weather Service San Diego California 
922 PM PDT Mon Oct 16 2017 


Synopsis... 
weakening high pressure aloft will bring another hot day to the 
region on Tuesday. A weak trough may trigger an isolated 
thunderstorm on Wednesday, but the chances are small. The trough 
will bring cooling Thursday and Friday along with a deeper marine 
layer and gusty west winds in the mountains and deserts. There is 
the potential of a Santa Ana wind event and heat wave across 
southwest California beginning next Sunday and lasting into the 
middle part of next week as high pressure builds. 


&& 


Discussion...for extreme southwestern California including Orange... 
San Diego...western Riverside and southwestern San Bernardino 
counties... 


It was another hot day away from the beaches with temps well into 
the 90s and even some triple digit readings. Two record highs were 
reached today - Ramona hitting 99f degrees (previous record 96 in 
1997) and El Cajon hitting 101f degrees (previous record 100 in 
1997.) A tranquil evening prevails and will continue overnight. 


The flow aloft will become more zonal on Tuesday, but surface 
cooling will be minor, and tomorrow will be another very warm day 
with temperatures 10-15 degrees above average west of the 
mountains, and 5-10 degrees above average in the mountains and 
deserts. 


On Tuesday night and Wednesday a couple of weak Pacific shortwaves 
traverse central and Southern California. The first wave passes over Santa 
Barbara and San Bernardino County Tuesday night. Moisture advection 
is not in phase with this first shortwave and that will limit the 
potential for elevated convection. Deeper moisture is available with 
the second wave on Wednesday. The WRF point sounding forecast shows 
near saturation in the 400-600 mb layer resulting in a precipitable water of 1.57" 
Wednesday afternoon. Most unstable cape values range from 200-400 
j/kg Wednesday, and the NAM high level total totals reach 31-33, 
which has historically been the trigger values for isolated to 
scattered thunderstorms. Have introduced a slight chance of 
thunderstorms on Wednesday to the mountains and parts of the deserts 
and valleys. This will be likely be adjusted over the next few 
shifts as more hi-rez model data becomes available. Precip amounts, 
if any, will be light with most of the precip evaporating in the dry 
subcloud layer. Best chances for measurable rain will be in the 
mountains. 


On Thursday and Friday a broad longwave trough moves into the 
Pacific northwest where rain is likely. Farther south in our area 
the precip chances are minimal, but the trough will bring cooling, a 
deeper marine layer, and stronger onshore flow with gusty afternoon 
and evening west winds in the mountains and deserts. 


On Saturday the trough moves east and a strong ridge of high 
pressure will build over the west. This Santa Ana weather pattern 
will bring another round of hot, dry weather and elevated fire 
weather concerns early next week. 


&& 


Aviation... 
170400z...few-sct clouds at or above fl200 and unrestricted vis through 
Tuesday evening. 


&& 


Marine... 
no hazardous marine weather is expected through Thursday. Winds will 
increase to around 20 kt or more on Friday as a front traverses the 
region. 


&& 


Beaches... 
a south southwest swell from 200 degrees will peak Tuesday-Wednesday 
at 3-4 ft/16-18 seconds. This will produce 3-6 ft surf with sets to 
7 ft north of del Mar. Highest surf will occur along southwest 
facing beaches. Strong rip and longshore currents will accompany the 
elevated surf. Swell and surf will begin to lower on Thursday. A 
beach hazard statement is in effect for elevated surf and strong rip 
currents Tuesday and Wednesday. 


&& 


Fire weather... 


Light winds, hot weather and low humidities will continue through 
Tuesday. The relative humidity will start to increase and temperatures start to 
cool Wednesday and Thursday as the ridge aloft weakens and onshore 
flow and the marine layer slowly return. A weak disturbance aloft 
interacting with increasing midlevel moisture may trigger an 
isolated thunderstorm Wednesday. Measurable rain is unlikely except 
in the mountains, where 0.01-0.10" are possible with any storms that 
do form. 


Thursday and Friday...onshore winds will be increasing over 
mtn/desert areas and become strong and gusty from the west with 
increasing relative humidity driving well inland. 


Outlook...Saturday looks to be a transition day from onshore flow 
to offshore. A Santa Ana wind event and heat wave looks possible 
beginning on Sunday and lasting through at least next Wednesday. 
We will continue to closely monitor this potential in the coming 
days. 


&& 


Skywarn... 
Skywarn activation is not requested. However weather spotters are 
encouraged to report significant weather conditions. 


&& 


Sgx watches/warnings/advisories... 
California...beach hazards statement from 6 am PDT Tuesday through Wednesday 
evening for Orange County coastal areas-San Diego County 
coastal areas. 


Pz...none. 
&& 



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