Harvey LA Forecast

Current:

I started by looking at the GOES-W water vapor satellite imagery from unisys weather over the past 12 hours. This shows the current large-scale pattern in the region. A large upper-level low is moving into central CA with strong PVA bringing stormy weather to most of the state. The radar agrees with this as there is rainfall across a large area of the state. An spc sounding from 12Z today actually shows some CAPE (albeit low values) due to the colder air aloft associated with the upper low. Thunderstorms cannot be ruled out especially this afternoon; surprisingly enough some storms may actually have small hail due to low freezing levels. Although the surface flow is out of the south, the winds throughout most of the upper-levels are westerly in accordance with the upper low. This indicates that there may be some enhanced vertical motion on the upslopes of the mountain ranges to the northwest of Los Angeles (from Santa Barbara to Greenfield) where the mountain range is oriented more north-south. Temperatures are expected to be relatively low with clouds and rain in the area and the deep upper-trough in the western CONUS. Highs today will be near 60 across much of the coast with much cooler temperatures at higher elevations.

GOES-W satellite from http://weather.unisys.com/satellite/sat_wv.php?inv=0&t=cur&region=we

Radar image from the National Weather Service radar site near LA

Short-Range: Tonight, Tomorrow, and Sunday

Precipitation should remain in southern California for much of the night until it finally shuts down tomorrow morning due to loss of forcing associated with the vort max moving off to the east. Cold overnight lows into the 40s should be expected especially in areas where the clouds move out overnight. Looking at the satellite loop, a ridge is located behind the upper trough indicating a warmup this weekend. The NAM, GFS and ECMWF show the progression of that upper low to be fairly slow with northwesterly flow across much of southern CA tomorrow. I used NCEP to view the NAM and GFS models and the Penn State E-Wall site to view the European. Temperatures tomorrow may be slightly higher due to more solar heating; however strong northwesterly winds will keep the highs near or just above 60. By Sunday, the ridge begins to build into the West, but most of CA is still experiencing northerly flow. Regardless, temperatures should be increasing with the ridge moving into the region (highs in mid 60s and lows near 50 in southern CA and cooler temps to northwest). Models are in good agreement as for the position of the ridge by Sunday.

500 mb Heights, Winds, Abs Vort for 12Z Sunday from today's 12Z NAM

http://mag.ncep.noaa.gov

Long-Range: Monday-Thursday

Models are in fairly good agreement as the ridge begins to move eastward and a zonal pattern begins to set in. The GFS is a little faster with the ridge weakening quicker resulting in westerly flow by 12Z Monday, whereas the NAM and European do not show this until 00Z Tuesday. With this pattern multiple days of fair weather are to be expected with comfortable highs and cool lows. Winds in the afternoon may be breezy as the upper winds may be mixed down to the surface, however with little moisture upslope precipitation is not expected. This pattern continues throughout the week as weak short waves move into the Northwest but do not affect CA too much. The GFS ensembles show the confidence there is in the model as good agreement is still in place through mid week.

GEFS 500 mb heights for 00Z Wednesday from NCEP

Verification:

Storm reports from Friday from www.spc.noaa.gov

Although there were a couple reports of severe wind there were no hail reports which were mentioned as a possibility in the forecast. Below to the right is an image from nmap2 showing surface obs and radar for the time period when the severe winds were reported. A thin line of storms appeared to be the cause of the strong winds in southern CA. The storms also brought a lot of rain; below to the left is an image of rain from 12Z the 13th to 12Z the 14th.

Total Precipitation from water.weather.gov/precip Radar image from nmap2 with surface observations

The rain kept the high Friday below the the expected values of lower 60s with observed highs in the mid 50s. Overnight lows were expected to be quite cold into the 40s and Los Angeles experienced a record low Friday night of 44F. Highs on Saturday were higher with more sunshine into the low 60s as expected.

Looking at the models, the timing of the trough coming through on the weekend was well forecasted as the models were in good agreement and the satellite matches well with the models. The models began to diverge Monday as for the strength of the incoming ridge; however with the zonal pattern setting in this slight disagreement did not last long. The models also did well on the precip the ocurred Friday into Saturday. The GFS did have an area of over an inch of precipitation southeast of LA which did not quite happen.

NAM Precip

GFS precip