Sloppy Storm for End of Week Will Bring “Mixed Bag” to Area

Thursday evening into Friday we will see a developing low pressure system begin to “bomb” off the New England Coast, bringing with it rain, snow, wind, and almost everything in between.

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Moisture from the gulf of Mexico (seen above at 48 hours, 00z Thursday) will begin to ride a quickly deepening trough late-week. As the storm rides up the coast late Thursday, all eyes are focused on the position and strength of the rapidly intensifying low. As of the latest runs, the NAM is trending slightly east of the ECMWF as of 06z Friday. The GFS seems to be more progressive than the other two, and also seems to be underestimating the QPF we often to see with these moisture-rich systems.
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Here, we see the bombing low’s placement, riding a negatively-tilted trough as of 06z Friday, according the Euro. It is always fascinating to see how quickly these storms explode off the coast: between 18z Thursday and 06z Friday, the low deepens from 1004 hPa to 976 hPa(!) according to the most recent NAM run. Even though this will not be a long-duration Nor’Easter, we will still see ample moisture, with QPF totals around 1-2″ for most of Central and Eastern MA (higher totals further east). However, the GFS, being more progressive with the storm, does not have nearly as much precip. falling over Southern New England because the storm is not fully developed at the time of impact. It is the outlier in that sense.

qpf1Now, the question on everyone’s minds: will it be rain or snow? Well… latest model trends have shown a more inside track, rather than the classic “Benchmark Blockbuster”. In fact, the GFS and the ECMWF both show temperatures around and within the I-495 corridor above freezing up to 925-hPa! This seems to be a snowstorm for interior Central New England, and Northeastern New England, the way the models are currently trending. Here’s a peek at the Euro’s snowfall projection through 1 PM ET Friday. Keep in mind… The Euro is the “snowiest” solution, as of now.

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Along the coastal plain, surface winds will likely gust 30-40 mph, with up to 50 mph possible on the Cape and Islands, as well as possibly the North Shore, as intense cyclogenesis occurs.

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It can’t be stressed enough: minor changes in the rain/snow line will have SIGNIFICANT impacts on our area. As is stands now, there is potential for 3-6″+ (possibly much more) outside of I-495, 2-4″ inside I-495, and even less south and east. Key word: Potential. If the storm slows down and trends further East, totals up to (and over) 12″ would not be out of the question for portions of Central and Northern MA, as well as some locations in inland NH and (particularly) interior Southern and Central Maine.

Stay Tuned to UML Weather Center for future updates because these fine details WILL change!

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Bitterly Cold Air in Store for the End of the Week Will Bring Dangerous Wind Chills to Our Region

A blast of arctic air will encompass the region just in time for the end of the work week. It’s pretty uncommon to see low temperatures flirting with negative digits before the first day of Winter, but this is exactly the case as we had into the early morning hours on Friday. An unusually deep (for this time of year) trough will be entering New England Thursday into Friday, allowing arctic air to funnel down from the north.

00z NAM 500mb.pngWe see the trough axis here on Wednesday’s 00z NAM run positioned over Western NY/PA, poised to enter New England in the coming hours. The leading cold front will usher in the cA air mass. This is in excellent agreement with the 12z NAM run from Tuesday, with the exception being that the newer run was very slightly more progressive with the trough entrance into our region by 12z Thursday. To put this into perspective, these 500-hPa heights are so low for this time of year that they are almost, if not more than, 3 standard deviations below average climo!

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So, what can we expect for surface temperatures Friday Morning? As of the latest SREF, by 09Z Friday morning (4 AM Eastern) temperatures dip down to -13 to -14C (between 7-8F) for Northeastern MA! Keep in mind, too, this is not the coldest air of the day; that won’t happen for another few hours later… The SREF plot below not only illustrates the mean temperatures, which stand out the most, but another key factor: decent agreement among ensemble members in the forecast, even at 66 hours out from initialization! We note the spread to only be 2-3 degrees over New England. This allows us to place higher confidence in the model’s forecast for this parameter. In fact, throughout the entire event, there is very little spread among SREF members! In terms of operation model run comparison, the newest NAM and GFS are both in good agreement for Friday morning SFC temperatures, with the NAM being slightly colder than the GFS in Eastern MA, at least at 12z Friday.

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Speaking of the “entire event”, how long is this supposed to last? Well, the trough seems to be taking its time enveloping us before the flow aloft transitions to more zonal in nature, and as such, we do not expect to see a quick bounce-back to more seasonable temps. The latest GFS does not have sfc temps. above freezing from just after 18Z on Wednesday all the way through a little before 18z Saturday. GFS meteogram for KLWM (Lawrence, MA) is below.

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We also see 10-meter wind speeds illustrated in the meteogram. Let’s talk about that. Overnight Thursday and into Friday morning, we will see a very well-mixed BL and a potent Low-Level Jet. If we are able to mix down some of the winds from the jet, we will see the potential for very strong, possibly damaging wind gusts at times. Below is the low-level jet position and magnitude at 06z Friday morning, according to the GFS. Note the area of 40-50kt winds over New England which, if mixed down, could cause damaging gusts at the surface at times. It would not be surprising to see some reports of downed branches, especially for Southern New England, late Thurs. into Friday. Expect Wind Advisories to be posted for MA, and maybe even High Wind Watches and Warnings possible, especially Southeast MA and CC&I.

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These powerful winds, combined with the arctic air entrenched in the region, will pave the way for very dangerous wind chills Friday morning. Anybody caught outside will be at-risk for wind chills that could be well below zero at times.

Lastly, on Friday, there is the potential for some snow squall activity, thought it would probably be short-lived (though, possibly moderate to heavy at times) and likely confined mostly to southeastern New England. Greatly reduced visibility is possible, but it would be surprising to see more than a few inches of snow, based on current model QPF. The tricky part is whether or not there will be enough moisture to work with; right now the synoptic set-up is not really ideal for a classic snow squall event across the region.

The next storm system will enter the picture early this weekend and will displace the cA airmass, likely bringing a change-over to liquid precip. thanks to warmer air advecting into the region. Stay tuned for more details on that system.

Messy Forecast For the End of the Weekend

You have all probably already heard about the potential for our first Plowable Snow Storm of the season, but what you haven’t heard is what we can expect here in our Neighborhood. This system is still three days out and hasn’t fully formed yet in the Midwest, so the data is a bit sparse, but we have some pretty good ideas along with the model guidance to go off of. First off, here is one model’s suggestion as to what we will see for Sunday Night and through the day Monday.

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From the looks of it right now, the Low Pressure system that will be bringing our precipitation will pass its center over the Great Lakes Region. In meteorology, we call that an inside runner. Usually with an inside runner, we start off with some snowy precipitation as there is still cold air in place, but as the storm tracks to the west of New England, the warm front from the Low Pressure System ends up passing directly over New England allowing warm air to intrude the region and cause the snow to change over to a mix and rain. However, this storm looks like it could end up being a special case, so let’s take a look at a few things.

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This image is from an analysis of Weather Types (WTs) (Weather Patterns seen during a season) in the Northeast by Roller et al (2015). Right now, we are in WT 1 as High Pressure is dominating the Southeast U.S. This storm can be seen as the Low Pressure system over the Midwest. This pattern is mostly seen during a (-PNA) phase, which is what we are seeing now. PNA is a form of teleconnection, which represents the low-frequency tropospheric height variability, as classified in the study of Barnston and Livezey (MWR, 1987). Basically it affects our weather patterns throughout the year based on the position of the jet stream and other factors. This pattern then transitions to WT 2 which sees the Low Pressure system push to the East and North. Most storm tracks tend to go into the Great Lakes Region.

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A look at the current PNA and NAO Indices. A negative PNA and Neutral to Negative NAO suggest stormier weather for the Northeast U.S.

By the end of the weekend, we will see a negative PNA and neutral NAO. This usually suggests stormy weather for the Northeast, but also suggests that inside running storms are likely. That means we are most likely to see a changeover to some mix and rain at some period during the day on Monday as warmer air is pulled into the region. graphic.aspx.png

Above you will see a map of the vorticity in the upper atmosphere. When we look at vorticity, we look for the regions of bright colors, indicating a vorticity maximum. That usually suggests somewhere around the area there will be rising air and the potential for some precipitation (this is one of many factors that influence whether or not precipitation will occur). Looking at this, we see the maximum that is near the center of the Low around the Great Lakes so there is expected precipitation to the west. There is another interesting area around Virginia that suggests some precipitation off the East Coast. There is suggestion that this could be picked up by the low pressure system and form a secondary low off the coast of Massachusetts. That would cause enhanced precipitation over Southern New England leading to higher amounts by the end of the day Monday.

This storm needs to be monitored closely during the next couple of days to fully see what will happen, but either way, expect a messy start to next week!