Author Archives: Scott Lindstrom

Dense Fog over Missouri and Illinois

Dense Fog Advisories were issued before sunrise on 5 July 2016 by the National Weather Service Offices in St Louis Missouri and Lincoln Illinois (link). This post compares GOES-R IFR Probability fields with Brightness Temperature Difference Fields for that event. At 0500 UTC, the Brightness Temperature Difference Field had a very strong signal over central/northern Illinois. This region of mid-level stratus was de-emphasized by the GOES-R IFR Probability fields because of a lack of low-level saturation in the Rapid Refresh Model Fields. At 0500 UTC, IFR Probability Fields show an increase in values over the lower Ohio River Valley.

GOES-13 Brightness Temperature Difference Field (3.9 µm – 10.7 µm) and GOES-R IFR Probability Fields, 0500 UTC on 5 July 2016 (Click to enlarge)

At 0700 UTC, below, an axis of higher probabilities has developed northwest to southeast across central Missouri; in addition, IFR Probabilities are increasing slowly over central Illinois to the south of the mid-level stratus that persists over east-central parts of Illinois. IFR Conditions are present at stations over central Illinois (Springfield — KSPI — and Litchfield — K3LF, for example)

GOES-13 Brightness Temperature Difference Field (3.9 µm – 10.7 µm) and GOES-R IFR Probability Fields, 0700 UTC on 5 July 2016 (Click to enlarge)

By 1000 UTC, below, when Dense Fog Advisories have been issued, a strong brightness temperature difference signal is present over much of northern Missouri, but mid-level clouds over southeast Missouri prevent a strong signal from occurring there where fog is occurring. IFR Probability Fields’ use of Rapid Refresh information mitigates this lack of satellite observations. IFR Probability fields at 1000 UTC show a signal over much of southern Illinois where IFR Conditions are widespread.

GOES-13 Brightness Temperature Difference Field (3.9 µm – 10.7 µm) and GOES-R IFR Probability Fields, 1000 UTC on 5 July 2016 (Click to enlarge)

Use IFR Probability fields as a tool to become situationally aware to the development of lowered ceilings and reduced visibilities. There are many times when Brightness Temperature Difference fields cannot tell the entire story — when multiple cloud layers exist, for example, or when mid-level stratus is present. A slow increase in GOES-R IFR Probability will often suggest lowering ceilings/reduced visibilities before Brightness Temperature Difference fields do.

Fog/Low Ceilings over Southwest Georgia

GOES-R IFR Probability fields on 27 June 2016 at 0200, 0400, and then hourly from 0700 through 1300 UTC (Click to enlarge)

Late-day thunderstorms on 26 June 2016 set the scene for the development of fog overnight over southwestern Georgia. The animation above shows the GOES-R IFR Probability fields. An enhancement in the fields that is initially driven by Rapid Refresh Model data showing near-saturation at low levels is apparent at 0200 UTC. As clouds associated with the departing convection dissipate, satellite data could also be used as input into the IFR Probability fields. The toggle below of GOES-13 Brightness Temperature Difference fields (3.9 µm – 10.7 µm), at 0200 and 0400 UTC, shows the appearance of low-level clouds as mid-level and higher clouds (dark in the enhancement used) dissipate. By 0400 UTC, when satellite pixels finally start to suggest low clouds, fog had already started to develop. IFR Probability fields gave an early alert to the possibility of fog development on this day that was not possible from satellite data alone.

GOES-R Cloud Thickness Fields can give a hint to when radiation fog, as in this event, will dissipate in accordance with this scatterplot. The image below shows the GOES-R Cloud Thickness at 1030 UTC, the last field computed before twilight conditions (indeed, the boundary showing that boundary is readily apparent over eastern Georgia), with values exceeding 900 feet in some places over southwest Georgia. Based on the scatterplot, that suggests a dissipation time of just over 2 hours (based on the best fit line, but note the scatter in dissipation times associated with cloud thicknesses of 900 feet: just over an hour to almost 4 hours!) so clear skies would be expected by 1300 UTC. The animation of visible imagery, here, shows that fog persisted just a bit longer than that, dissipating shortly after 1400 UTC. GOES-R Cloud Thickness field is an empirical relationship between 3.9 µm emissivity and cloud thickness that is based on SODAR observations off the west coast. The scatterplot was created based on past observations limited to the southeast part of the US and parts of the Great Plains.

GOES-13 Cloud Thickness, 1030 UTC on 27 June 2016 (Click to enlarge)

Low Ceilings and reduced visibilities over the Ohio Valley

Surface Observations at 1200 UTC on 24 June 2016 (Click to enlarge)

A screen capture from this site at 1215 UTC on 24 June 2016, above, shows IFR Conditions (Red) and Low IFR Conditions (Purple) over the upper Ohio River Valley and surrounding states. The IFR Probability field for the same time, below, shows high probabilities in roughly the same regions that have IFR or Low IFR conditions. The Brightness Temperature Difference field, also displayed in the toggle below, gives little information at this time of day. A benefit of the GOES-R IFR Probability field is that it contains a coherent signal through sunrise.

GOES-R IFR Probability fields and GOES-13 Brightness Temperature Difference Fields (3.9 µm – 10.7 µm) at 1215 UTC on 24 June 2016 (Click to enlarge)

The toggle at 0915 UTC, below, before sunrise, shows a second benefit of IFR Probability fields: a useful signal in regions with cirrus clouds. High clouds, of course, prevent GOES-13 from viewing the development of fog/low stratus near the surface. The Rapid Refresh model data on low-level saturation that are part of the IFR Probability Field computations give quality information in regions of cirrus. In the example below, developing IFR conditions are depicted (the yellow enhancement that shows IFR Probabilities around 40%) over much of northern Kentucky and southern Ohio. This is under a region of cirrus (black in the enhancement used for the brightness temperature difference) north of a convective system that sits over southeastern Kentucky and eastern Tennessee.

GOES-R IFR Probability fields and GOES-13 Brightness Temperature Difference Fields (3.9 µm – 10.7 µm) at 0915 UTC on 24 June 2016 (Click to enlarge)

The waning full moon provided ample illumination for the Suomi NPP Day/Night Band Imagery, shown below, from 0736 UTC on 24 June 2016. The cirrus shield, mid-level clouds and developing valley fogs are all apparent.

Suomi NPP Day/Night band imagery, 0736 UTC on 24 June 2016 (Click to enlarge)

IFR Conditions over Maine

GOES-R IFR Probability, and surface plots of ceilings and visibility, 0500-1215 UTC on 12 June 2016 (Click to enlarge)

IFR Probability fields, above (a slower animation is here), show high probabilities of IFR Conditions over much of Maine, but a definite western edge is also present, moving eastward through New Hampshire and Vermont and reaching western Maine by 1215 UTC. The screen capture below, from this site, shows IFR (station models with red) and Low IFR Conditions (station models with magenta) over much of southern Maine at 1200 UTC on 12 June in advance of a warm front.

Careful inspection of the IFR Probability animation shows a field at 1000 UTC that is very speckled/pixelated. This likely results from cloud shadowing. The combination of a very low sun and multiple cloud layers resulted in many dark regions in the visible imagery that the cloud masking may have interpreted as clear regions. (Click here for a toggle between Visible Imagery and GOES-R IFR Probabilities at 1000 UTC).

Surface plot at 1200 UTC 12 June 2016. See text for details (Click to enlarge)

Low IFR Probability fields are also computed by the GOES-R Algorithms. Values are typically smaller than IFR Probability. Plots of Low IFR and IFR Probabilities at 0700 and 1215 UTC are shown below.

GOESR_IFR_LIFR_0700_1215_12June2016toggle

GOES-R Low IFR Probability and GOES-R IFR Probability, 0700 and 1215 UTC (Click to enlarge)

Power Outages

Two power outages 12 hours apart at UW-Madison CIMSS have impacted distribution of GOES-R IFR Probability fields. ~~It’s possible that products may not be smoothly flowing again until Monday 13 June.~~ Data are flowing as of about 0000 UTC on Saturday.

In the interim, users can find the products at the GEOCAT site. If you’re in the southeast US, near Atlanta, an experimental site that compares IFR Probability and Brightness Temperature Difference fields is here.

For over the Northern Plains

GOES-R IFR Probability Fields, computed from GOES-13 and Rapid Refresh output, 0215-1115 UTC on 1 June 2016 (Click to enlarge)

Dense Fog Advisories were issued over parts of Iowa and Minnesota early on 1 June 2016 (see map below). The fog developed over wet ground left in the wake of convection that moved through the region late in the day on 30 May/early on 1 June (Precipitation totals available here). GOES-R IFR Probability fields, above, show the two areas of dense fog developing. The region over Minnesota was characterized a lack of high clouds — the satellite could view the developing fog, and satellite parameters were included in the computation of IFR Probability. Consequently, the IFR Probability values were larger.

Fog over Iowa initially developed under mid-level clouds behind departing convection. IFR Probability fields in that case show a flatter distribution because horizontal variability is controlled mostly by model fields that are smooth; additionally, IFR Probability values are somewhat reduced because satellite predictors cannot be used. By 0815 UTC, however, mid- and high-level clouds have dissipated, and the satellite has a unobstructed view of the fog/stratus. Satellite predictors could then be used and IFR Probabilities increased, and the field itself shows more horizontal variability as might be expected from the use of nominal 4-km resolution satellite pixels.

Screen Capture of weather.gov website at 1129 UTC on 1 June. Dense Fog Advisories are indicated over eastern Iowa and northeast Minnesota (click to enlarge)

Fog over the Great Lakes

GOES-R IFR Probability fields computed with GOES-13 and Rapid Refresh Data, 1215 UTC on 26 May 2016 along with surface reports of Ceilings and visibilities (Click to enlarge)

High Dewpoint air (upper 50s and low- to mid-60s) has overrun the western Great Lakes, where water temperatures are closer to the mid 40s. (Water Temperature from Buoy 45007 in southern Lake Michigan). Advection fog is a result, and that fog can penetrate inland at night, or join up with fog that develops over night. The image above shows the extent of low visibilities over the upper Midwest and the IFR Probability field early morning on the 26th of May. Lakes Michigan and Superior are diagnosed as socked in with fog. A similar field from 1945 UTC on 25 May similarly shows very high Probabilities over the cold Lakes. Expect high IFR Probabilities to persist over the western Great Lakes until the current weather pattern shifts.

Brightness Temperature Difference Fields can also show stratus over the Great Lakes, of course, but only if multiple cloud layers between the top of the stratus and the satellite do not exist. Convection over the upper Midwest overnight on 25-26 May frequently blocked the satellite’s view of the advection fog. The toggle below, from 0515 UTC on 26 May, shows how model data from the Rapid Refresh is able to supply guidance on IFR probability even in the absence of satellite information about low stratus over the Lakes.

GOES-13 Brightness Temperature Difference Fields and GOES-R IFR Probability fields, 0515 UTC on 26 May 2016 (Click to enlarge)

Dense Fog over Southern Iowa

GOES-R IFR Probability Fields, hourly from 0315 through 1215 UTC on 18 May 2016 (Click to animate)

Dense Fog Advisories were issued by the Des Moines Forecast Office on 18 May as fog developed across southern Iowa. The IFR Probability Field animation, above, articulates where the fog is developing — most of the stations reporting IFR (or near-IFR) conditions are within the region of enhanced IFR Probability, and regions where IFR conditions did not develop show persistent low values of IFR Probability.

Compare the 1015 UTC image of GOES-R IFR Probability and the GOES-13 Brightness Temperature Difference (a traditional way of detecting the presence of low clouds/fog at night), below. Because the IFR Probability fields incorporate information from the Rapid Refresh Model about low-level saturation, IFR Probability screens out the region in central and northern Iowa where the Brightness Temperature Difference field suggests clouds might be forming (but where surface observations show scant evidence of cloud).

GOES-R IFR Probability and GOES-13 Brightness Temperature Difference (10.7 µm – 3.9 µm) fields, 1015 UTC on 18 May 2016 (Click to enlarge)

The fog that developed over Iowa was fairly thin and should dissipate quickly under a strong May sun. The GOES-R Cloud Thickness field from 1045 UTC, the last complete field before twilight conditions, below, shows values mostly less than 700 feet with a few pockets of 800-900 feet. The toggle below shows the GOES-R Cloud Thickness at 1045 UTC and the IFR Probability field 90 minutes later. Regions with the thickest fog at 1045 UTC do show a persistent signal in the IFR Probability field 90 minutes later.

Cloud Thickness and Dissipation Time

GOES-R Cloud Thickness, 0945 UTC on 5 May 2016. Note that Cloud Thickness is not computed in the northeast corner where twilight has begun. This is last scene for northern Ontario and Michigan. (Click to enlarge)

GOES-R Cloud Thickness can be used to make a first guess of when fog and low clouds will dissipate. This is done via a look-up table that is derived from this scatterplot. The y-axis on that plot is the last GOES-R Cloud Thickness field produced before sunrise, such as that shown above, and the x-axis is the number of hours after the plot that clearing is expected, GOES-R Cloud Thickness relates 3.9 µm emissivity to dissipation time based on SODAR observations from off the West Coast, and the scatterplot was derived mostly from observations over the southeast US; the thickness is that of the lowest water-based cloud field and it’s not computed where multiple cloud layers exist — near Geraldton north of Lake Superior, for example, where IFR Conditions are reported — or near sunrise/sunset. Values are between 1100 and 1200 feet over northern Lower Michigan, around 1200 feet over eastern upper Michigan, and around 1200 to as much as 1490 feet over northwest Quebec. The scatterplot suggests a dissipation time of nearly 4 hours, which would be 1345 UTC.

Imagery below shows low clouds persisting just past 1500 UTC. For this region where the sun angle is not as high as over the southeast US (where most of the observations used for the scatterplot creation were taken), burn-off took a bit longer. However, note that GOES-R Cloud Thickness did highlight the thickest clouds that took the longest to dissipate; so, although the scatterplot underestimated the time of dissipation, Cloud Thickness values did identify which regions would clear last.

A final note: GOES-R Cloud Thickness Dissipation times were computed for Radiation Fog Events, such as the one on 5 May 2016. Dissipation of other fogs that create IFR conditions — Advection Fog, Tule Fog — will not be forecast well with this technique.

GOES-13 Visual (0.63 µm) imagery, 1115-1515 UTC on 5 May 2016 (Click to enlarge)

Low Ceilings and Reduced Visibilities over the Upper Midwest

GOES-R IFR Probability, 0515-1315 UTC on 26 April, with surface observations of ceilings and visibility (Click to enlarge)

GOES-R IFR Probability fields, above, expand southwestward across the upper midwest as ceilings lower and visibilities reduce. The fields offered quick guidance on where the lowest ceilings were occurring and how the field of low clouds was evolving. After sunrise (1215 and 1315 UTC imagery), IFR Probability values increased but continued to show a coherent signal over the region of lowest ceilings and smallest visibility.

The Brightness Temperature Difference fields for the same times, below, have structures that have echoes in the IFR Probability fields. The depiction of low ceilings and visibilities associated with the largest brightness temperature difference values (the deepest orange-red in the enhancement) is lost, however, as reflected 3.9 µm radiation alters the brightness temperature difference field. By 1315 UTC, the end of the animation, only the IFR Probability field is giving useful information about the low ceilings and reduced visibilities.

GOES-13 Brightness Temperature Difference (10.7 µm – 3.9 µm) Fields, hourly from 0515 through 1315 UTC, 26 April 2016 (Click the enlarge)

GOES-R Fog Product Examples

Fog detection fusing GOES, Terra/Aqua or Suomi/NPP Satellite data with Model output