Meiszberg Forecast 8

...Forecast Discussion...

Today through Thursday April 19

Current Weather Analysis

Recent surface observations, combined with radar and satellite imagery depicts the onshore movement of a low pressure center now western Nevada which is associated with a large upper level trough. This synoptic pattern is providing much of Southern California coast with southwesterly winds along with cooler temperatures. Current composite radar animations show light to moderate precipitation following this flow and reaching the Sierra Nevada mountain range where the rain is changing over to snow in the northern part of the range.


An analysis of 500 mb height anomalies for the northern hemisphere depicts a clear ridge-trough-ridge pattern across the Pacific as of 600 UTC today. This setup provides quick summary of the weather patterns as today's 12Z GFS run shows a continuation of this pattern as it tracks eastward. In the short-term, models are in close agreement on the chance for precipitation continuing through tonight as the current trough continues to progress eastward along with an associated cold front. This has lead to the issuing of winter storm warnings along much of the Sierra Nevada range while light to moderate rain remains the concern for the southern half of the state. The associated 500 mb trough begins to shift towards a negative tilt through the night shown in the GFS forecast. The vorticity maximum at the base of this trough will provide some of key forcing for this rain event as it swings through the region. Good upper level support is also seen in the 250 mb jet stream as the LAX region is in close proximity to the left exit region of the jet core wrapping around the base of the trough. Meanwhile, radiational cooling overnight will be limited by the associated cloud cover and act to keep low temperatures around area in the upper 50's. As this disturbance shifts out of the region tomorrow the ridge currently over the Hawaiian Islands will move in and begin to provide drier and more sunny conditions. This synoptic pattern shift will also be accompanied by greater wind speeds as the pressure gradient increase across much of the state. These surface winds will first become northeasterly tomorrow afternoon and then weaken as they become northwesterly on Sunday. This surface high pressure and associated upper level ridge will dominate much of the states weather pattern beginning this weekend and through Tuesday. Thus clear skies are expected for the week with high temperatures near the mid-70's based on extended range GFS statistics.

The next pattern shift will come the middle of the week as the next upper level trough moves into the northwestern U.S.. This feature however appears to remain well north of the LA area with conditions remaining dry through week as a surface high pressure center is forecasted to persist over the Pacific according to GFS.

Verification (April 17th)

Observations and radar for last Friday (Apr 13) and Saturday (Apr 14) are in good agreement with what was forecasted for the short term. The greatest precipitation amounts occurred in the late afternoon and evening on Friday. This rain then cleared out for LAX by very early on Saturday. Some of the thunderstorm associated with this were associated with wind reports in San Diego.

0000 UTC Apr 13 2012 Composite Radar 0000 UTC Apr 13 2012 NAM40 PMSL/6hr Precip

From NWS Climate Reports:

April 13th (Friday)

High: 59

Low: 50

QPF: 0.44 inches (broke record of 0.41 inches)

April 14th

High: 60

Low: 45 (tied record)

QPF: 0.00 inches

April 15th

High: 62

Low: 48

QPF: 0.00 inches

April 16th

High: 67

Low: 52

QPF: 0.00 inches

Overall, the short term part of the forecast verified well with the rain event occurring Friday and clear conditions for the weekend as the upper level ridge moved onshore. The NAM was rather consistent with the progression of the overall upper level pattern through Monday 00Z but was slowed down slightly based on a comparison of the 12Z Apr 13, 12Z Apr 14, and 12Z Apr 15 runs. This slow down is best seen in the movement of the shortwave trough out ahead of the ridge.