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High Pressure Doesn’t Always Mean Clear Skies

One of the first things that meteorologists learn in their Intro to Weather class is that low pressure means cloudy skies and rain while high pressure means clear skies and sun. Of course, the second thing that meteorologists learn is that weather often doesn’t behave in a textbook way. The... Read More

One of the first things that meteorologists learn in their Intro to Weather class is that low pressure means cloudy skies and rain while high pressure means clear skies and sun. Of course, the second thing that meteorologists learn is that weather often doesn’t behave in a textbook way. The southeastern United States today is a good example of that.

Surface map for 1200 UTC on 11 February 2025 from the NOAA National Weather Service Weather Prediction Center.

Looking just at the surface observations from the Weather Prediction Center, one would think that the Atlantic southeast is under fairly quiescent conditions. A strong high pressure system is centered over southeastern Pennsylvania, but an arm of this high extends south all the way into northeastern Georgia. A stationary front parallels the northern border of Florida. Even the low pressure system in northern Mississippi is quite weak, with a central pressure that is higher than the average mean sea level pressure. All in all, it looks to be a fairly ordinary day.

But the satellite on SSEC’s Real Earth tells a different story. Here’s a loop of true color imagery from GOES-16 this morning.

Loop of GOES-16 True Color RGB from the morning of 11 February 2025

Obviously, the Atlantic southeast is dominated by clouds, which seems to stand in opposition to what the surface map indicates should be happening. Some of the cloudiness is due to overflow from the midwestern cold front. But a good deal of the cloudiness is caused by the cyclonic flow around the high pressure in the northeast. The flow is coming from over the ocean, where is becomes laden with moisture. As it moves ashore, it runs into the southern extend of the Appalachian Mountains where orographic uplift forces it aloft and clouds and rain ensue.

One of the localized phenomena that arises from this kind of flow in northeastern Georgia has been dubbed the “wedge.” In wedge events, a shallow, stable layer of cold air forms due to cold air damming against the Appalachians. A sounding from the 1500 UTC RAP analysis shows this clearly:

RAP analysis profile from northeastern Georgia at 1500 UTC on 11 February 2025. From the College of DuPage NEXLAB

This time of year is historically home to the strongest cold air damming events in this region, and their effects can be significant. Low level inversions like the one seen today can contribute to hazardous wintertime precipitation like freezing rain, as snow formed in a saturated layer aloft melts as it falls through the elevated warm layer then freezes again when it contacts the surface. In April 2023, a wedge event received national attention: the Masters golf tournament was interrupted by a backdoor cold front that formed from the wedge of cold air flowing southward along local terrain. During that event, winds exceeded 30 mph, trees were felled near spectators, and play was interrupted due to these hazardous conditions.

Days like today are a reminder that it’s always a good idea to get a bird’s-eye view of the weather using the wealth of observations that satellites can provide.

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Leeside cold frontal gravity waves over the Southern Plains

GOES-16 (GOES-East) Upper-level Water Vapor (6.2 µm) and Mid-level Water Vapor (6.9 µm) images (above) displayed leeside cold frontal gravity waves (reference) as they propagated southward across Oklahoma, Texas and far eastern New Mexico on 8th February 2025.In a corresponding animation that included plots of surface wind barbs, peak wind gusts and... Read More

GOES-16 Upper-level Water Vapor (6.2 µm) and Mid-level Water Vapor (6.9 µm) images, from 1511 UTC on 8th February to 0001 UTC on 9th February; rawinsonde sites are plotted in red [click to play MP4 animation]

GOES-16 (GOES-East) Upper-level Water Vapor (6.2 µm) and Mid-level Water Vapor (6.9 µm) images (above) displayed leeside cold frontal gravity waves (reference) as they propagated southward across Oklahoma, Texas and far eastern New Mexico on 8th February 2025.

In a corresponding animation that included plots of surface wind barbs, peak wind gusts and frontal analyses (below), it could be seen that the gravity waves were generally present along or just behind the western portion of the surface cold front — while farther to the east, the gravity waves advanced ahead of the cold front.

GOES-16 Upper-level Water Vapor (6.2 µm) and Mid-level Water Vapor (6.9 µm) images, with plots of hourly surface wind barbs (white), 30-minute peak wind gusts (cyan/yellow) and 3-hourly surface front analyses (beige) from 1511 UTC on 8th February to 0001 UTC on 9th February [click to play MP4 animation]

Plots of GOES-16 Water Vapor (6.2 µm / Band 08 and 6.9 µm / Band 09) Weighting Functions derived using rawinsonde data from Fort Worth, Texas (KFWD) (below) indicated that the peak contribution of upwelling radiation for both spectral bands was originating from the middle troposphere (at pressure levels of 516-460 hPa) — with no contributions from the surface.

Plots of GOES-16 Water Vapor (6.2 µm / Band 08 and 6.9 µm / Band 09) Weighting Functions, derived using rawinsonde data from Fort Worth TX at 1200 UTC on 8th February and 0000 UTC on 9th February [click to enlarge]

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GOES-19 Upper-level Water Vapor (6.2 µm) images, with plots of hourly surface wind barbs (red) and gusts (gray/orange/red), from 1401-2331 UTC on 8th February [click to play animated GIF | MP4]

The leeside cold frontal gravity waves were also apparent in GOES-19 (Preliminary/Non-operational) Upper-level Water Vapor (above) and Mid-level Water Vapor images (below).

GOES-19 Mid-level Water Vapor (6.9 µm) images, with plots of hourly surface wind barbs (red) and gusts (gray/orange/red), from 1401-2331 UTC on 8th February [click to play animated GIF | MP4]

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Grampians bushfire in southeast Australia produces a pyrocumulonimbus cloud

10-minute JMA Himawari-9 AHI “Red” Visible (0.64 µm), Shortwave Infrared (3.9 µm) and “Clean” Infrared Window (10.4 µm) images (above) showed the formation of a pyrocumulonimbus (pyroCb) cloud that was spawned by a bushfire in Grampians National Park in far southeast Australia on 4th February 2025. The pyroCb exhibited cloud-top 10.4 µm... Read More

JMA Himawari-9 “Red” Visible (0.64 µm, top), Shortwave Infrared (3.9 µm, center) and “Clean” Infrared Window (10.4 µm, bottom) images from 0300-0510 UTC on 4th February, with plots of surface reports at Melbourne YMML [click to play animated GIF | MP4]

10-minute JMA Himawari-9 AHI “Red” Visible (0.64 µm), Shortwave Infrared (3.9 µm) and “Clean” Infrared Window (10.4 µm) images (above) showed the formation of a pyrocumulonimbus (pyroCb) cloud that was spawned by a bushfire in Grampians National Park in far southeast Australia on 4th February 2025. The pyroCb exhibited cloud-top 10.4 µm infrared brightness temperatures (IRBTs) in the -40s C (denoted by shades of blue to cyan), attaining a minimum IRBT of -49.9º C at 0430 UTC. This temperature roughly corresponded to an altitude around 11 km — not far below the tropopause — according to rawinsonde data from Melbourne: (plot | text). The pyroCb cloud drifted southeast, eventually passing just south of Melbourne Airport (YMML).

Himawari-9 True Color RGB images created using Geo2Grid (below) displayed the pyroCb cloud that was being transported southeastward — and dense smoke (shades of tan) was evident in the vicinity (and immediately downwind) of the Grampians bushfire.

JMA Himawari-9 True Color RGB images, from 0250-0600 UTC on 4th February [click to play animated GIF | MP4]

As a surface trough of low pressure was moving east-northeastward across the state of Victoria during that time period (surface analyses), strong southerly winds behind the trough axis (surface observations at Melbourne) helped to intensify the Grampians fire complex — and the pyroCb cloud developed just after the trough passed through the area. Himawari-9 Fire Temperature RGB images (below) revealed (1) the northward expansion of the Grampians bushfire following the ~0300 UTC trough passage (along with flare-up of new fires to the NW), and (2) the pyroCb formation just after the time of the trough passage (0340 UTC image). Note that in the Himawari-9 True Color RGB images shown above, the trough passage also initiated a northward transport of boundary layer smoke from the bushfire source region.

Himawari-9 Fire Temperature RGB images, from 2100 UTC on 3rd February to 1100 UTC on 4th February [click to play animated GIF | MP4]

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What’s the Weather going to be on …

“All weather is local” (and can change quickly) and satellites allow the monitoring of a range of phenomena on fine time and space scales. This includes, but not limited to air quality, aviation, convective initiation, the cryosphere, fire detection, fog detection, heavy rain, lightning, marine weather, tropical cyclones, volcanic activity, winter weather and more. There are a number of... Read More

“All weather is local” (and can change quickly) and satellites allow the monitoring of a range of phenomena on fine time and space scales. This includes, but not limited to air quality, aviation, convective initiation, the cryosphere, fire detection, fog detection, heavy rain, lightning, marine weather, tropical cyclones, volcanic activity, winter weather and more.

There are a number of online resources to better understand what the weather is going to be like at a given place and time. What resources to look at will depend on the location and length of the forecast ahead of time. For example, if the date is too far in the future, then climatology may be the best tool, while satellite imagery may be the best for a short-term cloud cover “nowcast”.

GOES-16 ABI animation over southern Wisconsin.

The best place to start might be your local NOAA NWS Weather Forecast Office web page (eg, MKX), reachable via a national map.

A screen shot of the top part of the MKX Weather Forecast Office web page.

Climatology

Years or Months before the event one is interested in, climatology shows what has happened on this date in the past. It’s best to look at not only the averages (for temperatures and precipitation), but also the extremes. Click on “Climate Graphs”. Or visit the CPC page.

MKX’s Climate Page

Click on “Normals” or “Records”, then chose the City and month of interest.

Madison’s Climate averages for early February.
Madison’s Climate records for early February.

Long-term Outlooks

There are a number of longer-term outlooks for temperature and precipitation, including 3 months ahead of time, as was as weeks ahead of time. Start by clicking the “Outlooks” tab.

Example of a seasonal (precipitation) forecast.
An example of a monthly (temperature) outlook.
An example of a 6-10 day (temperature) outlook.

Within a Week

A week ahead of a given event is covered by global forecast models. These models leverage many observations, including satellite observations. A few days ahead of a given event is covered by regional forecast models. These models leverage many observations, including satellite observations. In fact, satellites are the backbone of NWP observations. Guidance from both of these can be found on the WFO home page, or the hourly-weather graph.

The top part of the WFO MKX page, with current conditions and the forecasts.
A more detailed forecast, including the location the forecast is valid at.
An example of the hourly weather graph, which includes many parameters.

More on how to read the hourly weather graphs from the NWS.

Same day

The closer to the time of the event, the more one can view current weather, including satellite images. This includes integrated products like the research probability of severe weather or lightning, fires products.

LightningCast over a GOES-16 RGB composite imagery for July 16, 2024. (‘Click to Play”)

More on LightningCast.

Many states have a “mesonet” of surface observations, including in Wisconsin (maps).

More resources, such as satellite and radar information.

More satellite imagery from NOAA/STAR, UW/SSEC, UW/GeoSphere and CSU/CIRA.

Time of the event

While “looking out the window” is good advice, even better might be to go outside and check. Of course the weather can change quickly, so keep an eye on rapidly changing conditions. One can also check out a number of roof top cameras, for example those at UW/SSEC and AOS.

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