Tag Archives: mos

NYC New Year’s Day Detailed Forecast – Jan 1, 2019

NYC, Rain, Storms, Weather Update, , , , ,

A wet, windy New Year’s Eve in NYC will give way to a very mild day to start 2019. However, for reasons outlined below, I don’t believe that record warm temperatures will be broken tomorrow. Strong winds will be a distinct possibility, though, and as such the National Weather Service has issued a wind advisory for the area. I think sustained winds could reach 30 mph, especially early in the day.

My Forecast

High: 57°F | Low: 36°F | Max sustained winds: 30 mph | Total precipitation: 0.04″ –
verification for temperatures and precipitation will come from METAR data for the period between 1AM Tuesday and 1AM Tuesday (06Z Tuesday to 06Z Wednesday) at LGA. Wind speed verification will draw on the daily climate summary from the National Weather Service.

Verification

High: 60°F | Low: 40°F | Max sustained winds: 36 mph | Total precipitation: 0.02″ – this wasn’t a great forecast, aside from the precipitation. I was once again overly conservative on max wind speeds. Following layer mean wind analysis results would have yielded an almost perfect forecast here. For temperatures, I think the main issue was that the low level clouds I thought would materialize based on the forecast soundings available the day before just never came to be. This allowed high temperatures (and hence low temperatures) to become warmer than I forecast. In this case, perhaps it would have been prudent to consider that winds would be downsloping from the northwest, hence drying out somewhat at lower levels. This wind direction also favors clearing conditions following a cold front passage from the west.

Weather Prediction Center surface forecast for 7AM Tuesday

Synoptic Set Up

NYC will start the day off in the warm sector of a low pressure center tracking northeast through New England. This low will continue to deepen as it progresses east. The tightening pressure gradient will induce strong winds, and a trailing cold front will cross the region bringing about much colder air in its wake. Temperatures will peak early in the day and drop steadily through the day. The low temperature will occur late in the forecast period.

High Temperatures

Steady warm advection overnight with a warm front and warm sector associated with the parent low above will mean that temperatures increase to the mid-50s by day break. Forecast soundings from both GFS and MOS indicate, persistent, low overcast that I believe will be pivotal to keeping temperatures from going above 60°F. These low overcast clouds should block out enough sun to keep things a bit cooler than otherwise would be. Forecast soundings also show some upper level clouds that would help block additional sun. For these reasons, I’m siding with cooler NAM MOS guidance that calls for 55°F, and bumping up a couple degrees. Cold advection should take hold quickly in the afternoon in the wake of the trailing cold front. This will usher in westerly and northwesterly winds, which will downslope and warm a touch but will still overall bring colder air into the area. Once this cold advection takes place, temperatures should start to fall.

GFS forecast sounding for 9AM Tuesday, showing a well mixed layer up to about 825 mb and low clouds around 800 mb.

Low Temperatures

Because of the factors above, the low temperature should end up coming in late in the evening/overnight period going into Wednesday. I believe there’s not too much reason to deviate from MOS guidance which averages out to be about 35°F.

Max Sustained Winds

A well-mixed boundary layer is forecast to develop with pretty strong winds aloft in the late morning hours. A period of efficient downward momentum transfer should allow fast winds from above to mix down to the surface. Layer mean wind analysis suggests that winds could be as strong as 35 mph, however, I’m going a bit lower with sustained winds topping out at 30 mph since neither set of MOS guidance goes over 20 mph.

Total Precipitation

It’s clear at this point that the bulk of precipitation will fall before the beginning of the forecast period. However, both sets of MOS guidance, SREF and GEFS means all suggest some precipitation early in the overnight period. I’m going with a composite average of various sources at 0.04″.

WxChallenge Caribou, ME (KCAR) Day 7 High Temperatures – Nov 4, 2018

Cool Images, Outside of NYC, , , , , , , , , ,

What follows is a discussion lab that I wrote for this past week’s WxChallenge forecast competition as part of Penn State University World Campus’ METEO 410 capstone class in weather forecasting. I’m sharing this to give folks a glimpse into the forecasting process we’ve been learning, and because this discussion lab garnered some plaudits from my instructor for providing a really good analysis with attention to detail.

Model Guidance

12Z NAM MOS and 18Z GFS MOS (model output statistics) today agreed on a high temperature of 46°F for Day 7 (06Z Thursday to 06Z Friday). 00Z Wednesday’s NBM run was forecasting 44°F. 18Z EKDMOS shows ~46°F in the 50th percentile, with 50°F in the 90th percentile, and 41°F in the 10th percentile.

MOS forecasts October 31, 2018

National Blend of Models (NBM) – a consensus MOS product

EKDMOS (Ensemble Kernel Density MOS), producing a probabilistic forecast of severeal different variables. This shows maximum temperatures. The green bar shows the 10th percentile, red bar the 50th percentile, and blue bar the 90th percentile

Synoptic Set Up

By Thursday, the cold front of the occluded low that will bring precipitation Wednesday is forecast to have pushed through KCAR. During this frontal passage, winds will veer from the SSE towards the west. Winds are not forecast to be particularly strong, however, westerly winds would downslope a bit, enhancing wind speeds as well as warming temperatures a touch.

Weather Prediction Center (WPC) surface forecast for 00Z Friday November 2 (8PM Thursday, November 1, 2018)

Typically, we’d expect temperatures to be cooler behind a cold front due to cold air advection (CAA). Checking dynamical model forecast 2-meter temperatures, there’s not really evidence of large temperature gradients around KCAR. Even though winds will be blowing from areas of cooler temperatures towards warmer temperatures, the lack of a large gradient and low wind speeds do not suggest strong CAA.

It’s worth noting that the cold front appears to have anafrontal characteristics (precipitation behind the front seen in the WPC surface forecast) – this has implications on cloud cover behind the front. Both sets of MOS guidance show overcast conditions throughout the day. Forecast soundings suggest the main effect of the cold front is a drying out of the layer between approximately 900 mb to 600 mb initially, though by 18Z Thursday this layer dry layer tops out 700 mb (NAM has a smaller dry layer, between 900 mb and 750 mb – not pictured). Outside of this dry layer, clouds appear likely both near the surface and also from the top of the dry layer to as high as 200 mb (300 mb in NAM). In fact, it appears that the column above the dry layer will be saturated, and precipitation will be falling at upper levels during periods of the day, which explains why the dry layer shrinks from the top down as moisture works its way down through the column.

GFS forecast sounding

Closing Thoughts

Because of the likelihood of persistent, seemingly thick overcast during peak heating, I’m hesitant to side with the MOS consensus of 46°F, which I think is too warm. Even if precipitation doesn’t reach the ground, evaporational cooling may still be a factor. I think NBM’s 44°F is reasonable given the current data. I wouldn’t go too low into the low-40s because of warming impacts of downsloping westerly winds and the lack of any strong CAA.

Results

Subsequent MOS runs actually trended up, as high as 49°F. However, because of the factors outlined above, I continued to hedge down from MOS guidance, and submitted a finalized forecast of 46°F on the day. The actual high ended up being 45°F. Because I hedged down, I was able to minimize my error points for the day and ended up climbing to the top of the class leaderboard.